How to choose music as a swing DJ: Venn diagram
In the last couple years I've started DJing for swing dancers and have had the opportunity to mentor new DJs in my university's swing dance club. This is my model for explaining what sort of music to play:
Playing music you love makes DJing awesome.
Playing music your audience loves gets people dancing. This is about the audience response to what you're playing now — whether people are dancing, smiling, having a good time — not just about playing known favourite songs.
When you play music that fits both categories, everybody has a great time.
Venture outside the middle sometimes as needed: I don't like some floor-fillers but they're worth it for the energy. On the other hand, playing some songs that don't match the audience's taste helps people dance outside their comfort zone.
(Beyond this advice, I recommend Daniel Newsome's 2013 article: "How to be a better Swing DJ".)