Variety and Pacing

You want to give your dancers a contrast between high and low energy.  Obviously you don't want to play two tunes in a row for the same kind of dance, at the same tempo.  Have you ever danced to a big band that played three swing or foxtrot tunes in a row with the same tempo?  The bandleaders were clearly not dancers.

Be aware that dancers will want to catch their breath after a fast tune, so maybe follow it with a slower one.  And they don't want to be lulled to sleep with too many slow songs in a row.  But you don't have to strictly alternate fast and slow dances.  Some DJs like to build the energy over several dances, bringing the dancers higher and higher but without exhausting them (yet).  Similarly, a string of several quiet dances can effectively set a deeper mood while also building up a desire to be hit with a high-energy set.

The art is to find the perfect pace, without too many fast or slow dances in a row.  If you want a safe rule of thumb (which has exceptions), don't play more than one or two really fast tunes in a row, and don't play more than two or three slow tunes in a row.