IMHO a bridge gives the opportunity to look at the events in the song from a different perspective - eg how did things used to be?, what will happen in the future if "x" happens?
I completely agree with
@adamfarr that it will be much easier to write the bridge - if you still feel the song needs one (which is a judgement call depending on the content of the verse/chorus sections, the story you are telling, the structure of the song, the emotion of the song etc. etc.)
If the main part of your song is about how these 2 people came together (as suggested by Adam) then 2 suggestions for the bridge could be:
- what would have happened if they never met
- what will things be like 10 years from now
I hope this helps kick start your ideas in the right direction