On the one hand, it means someone is engaged with what you're doing. Or you could take it to mean that they're not satisfied with the songs that you've been playing. I'd say in the vast majority of cases, it's the former, but as one of many introverted singers, the alternative does creep into your head from time to time.
Myself, I sit somewhere in the middle of this spectrum, and slide left and right on any given night. It's fun to play something for someone generally, especially if they want to hear something a little different than the average person. But what gets frustrating is the monotony of the songs requested, or having to feel like you're at fault and have to apologize for not knowing a particular song. I am not a jukebox - I am a living, breathing person with likes and dislikes. I can't just have a file programmed into my system and have it there forever. Learning a song takes time, especially for someone like me who feels the need to not use a music stand and keep everything memorized. It takes listening to the song enough times to have the outline and lyrics and melodies and dynamics to engrain themselves in my mind before I can move on actually getting it performance ready. Therefore, I tend only to learn songs I enjoy.
An interaction I had with someone at a gig recently went something like this:
Her: Do you know any Justin Bieber?
Me: No I don't, sorry!
Her: Why not?
I generally dread when I see someone approach me at a gig while I'm playing, not because I hate taking requests, but I always assume the worst is coming - some scenario like this where I try to remain polite and respectful of someone's musical tastes, while I'm internally just getting really flustered. I don't do well with conflict, and I'm not one to sass back to someone unless they manage to really set me over the edge. I generally encourage folks to let me know if they think of anything else they might like, and I'll do my best to play it for them if I can. Unless they're relentless with requests that I don't know, I usually avoid handing them my repertoire list except as a last resort, because that usually ends badly for me. One year on St. Patrick's Day that happened, and rather than lessening my struggle, it turned into a drunk person coming up to me about 9 times while I was mid-song, often mid-line, to put in a steadily overflowing queue of songs he and his friends wanted to hear. One song in particular he did this to me three times before it was finished.
One of the strangest phenomena to me is the stalwart requests that everyone has. The Wagon Wheels and Wonderwalls of the world. The inevitable "do you know any Johnny Cash?" from people ranging age 9-99. Again, I'm not one to bash anyone's musical tastes, but it's just hard to grasp why, in the whole wide world of music, everyone always sees an acoustic guitarist and immediately wants to hear these songs. I have nothing against Johnny Cash, I've grown to like some of his stuff, I appreciate his importance in the world of music, but I don't really listen to him, and therefore don't have any of his music in my repertoire. I do know Wagon Wheel, I do know Wonderwall, and if you ask me (please don't...) I will play them for you, albeit somewhat begrudgingly. But at this point I've had to play them so many times that I will only do them by request anymore. At times in my life, I enjoyed both of those songs, but they've just been irreversibly run into the ground for me.
Are these really the songs you listen to all the time? I'm positive that more often than not that is not the case. You just want to be involved and make a request, and default to the go-to's that everyone knows, or that you assume everyone does. But the issue here is I don't want to be everybody. Not in a hipster kind of way, I am certainly far from that, I just want to be seen as an individual. I want people to go to the bar to hear Ryan Sweezey, not to hear some singer guy with a guitar. I've prided myself on building a huge catalog of songs I can play to please all ages while still being a bit out of the ordinary and true to my own likes and dislikes. I don't play songs nobody knows, I try to play the songs they do know but don't hear all the time, and save for the Johnny Cash fans of the world, I get as many compliments on my set lists as I do any other aspect of my performance. I've had regular spectators who reassure other folks that if I don't know their request, they shouldn't worry because they will hear plenty that they enjoy. But there are still those times when someone just gets so fixated on one particular thing that if I can't do it, I become the enemy.
Melding these scenarios together, there is one instance in particular that stands out to me. I was playing at a bar, and a girl came up to me and asked if I knew any Johnny Cash. I told her I was sorry but I didn't. She came back shortly thereafter asking if I knew She Talks to Angels by the Black Crowes. I told her I was sorry, I didn't, but I do love that song. At this point, she said "You need to brush up on your oldies." I played it off as best I could, but as someone who knows over 350 songs, about 75% of which came out long before I was born, it was all I could do not to fly off the handle here. I proceeded to then vengefully play several songs in a row I would consider oldies. She later came up and asked again if I would play She Talks to Angels. Now not willing to deal with this, I told her that no, in the past half hour while I've been playing other songs I have not learned this song. For the next hour or so, she would stare daggers at me from the bar while loudly singing the song she wanted to hear. When she wasn't doing that, I could hear her complaining to people around her how I was mean because I wouldn't play the song she wanted. Eventually she stopped, before coming up to me one last time near closing time to see if I knew Hot in Here by Nelly.....
In the end, at these types of gigs in particular, I'm being paid to give the people what they want, so it's really just about finding the balance between accepting the inevitable and being able to still enjoy what you're doing. Sometimes you just have to grit your teeth and push on through stuff you don't like playing. If you're a performer, you know this struggle, and you either embrace it or soldier through it. If you're a spectator, please don't think me bitter, just envision yourself in my shoes and try and be as respectful of your entertainer as they are trying to be to you. My goal here is not to complain, it's simply a statement of my observations. At the end of the day, I get to do something I love doing for a living. It's hard to call it work, but sometimes instances like these drain a bit of the enjoyment out of it. I'm going to accommodate you if I can, but I shouldn't feel like I'm inept or in the wrong for not knowing how to play something in particular.