In a music video the music needs to fit the action. Also the ideal time for a video is 3-5 minutes. Any longer and you may have trouble keeping a viewer's interest unless it's very good.
I like to actually base my video around my song selection, I also enjoy matching video clips to the beats in the song. Both bass drum beats, and tempos as well. ie if it's a slow part in the song I'll put in a slow dog fight scene or something else creative.
I usually do most of my videos quite fast paced though, to keep the energy.
As for actually choosing music, it just depends on what you want to put in it. Most of my music is from favorite songs. Just depends on what you think feels fit, it's your video after all!