Djs are all about playing happy uplifting songs but every now and then we get a request for something we know will kill the vibe.
As a DJ I want the dance floor full for as long as I can possibly manage. Most DJs love to play a good request, but sometimes people will come and ask for a song they like, but certainly won't get people dancing. Just imagine you've been rocking to a load of great Motown, Dianna Ross, Stevie Wonder, The Four Tops etc, then someone comes over and asks for the Moody Blues, Knights in White Satin or John Miles, Music was my First Love. Both good songs but they will only have a meaning to one or two people in the room. Worse still some sombre songs can have a detrimental effect on mood, especially if it's a song someone links to a lost loved one or an event they would prefer to forget. DJs already have a minefield of songs to avoid for many reasons, and spend a lot of time thinking about the psychological effect songs can have. We are trying to achieve one thing, positivity!
Also remember that the DJ could be working with a mandate from the client, a kind of vibe they have agreed to before the event. Personally I will try to fit in 95% of requests. I do filter some I find won't fit or aren't appropriate. if you do have a song which you think might be left field, perhaps give the DJ a reason to play it, some background info. And of course remember the DJ is a person, be nice to them and respect their decision.