The meaning is two-fold.

1. There will always be another girl/guy after you're done with one. Just like a bus stop, there's always one coming 15 minutes later.

2. A 'dry spell' is much like a bus stop. Sometimes there won't be a bus coming through for awhile, then all the sudden, BAM there's 4 buses. Same thing with a dry spell, you go through a long time of no action then BAM multiple people are interested in you.