Because everyone's got a story! And here is mine...
I was on a bus recently, quite a full bus. A wheelchair in the wheelchair space, one lady with a pram (unfoldable I think) in the buggy space. Bus pulled up at a bus stop, where two ladies are waiting with pushchairs, chatting to each other.
One lady gets on, parks her pushchair into the remaining buggy space. The second lady tries to get on, but the bus driver won't let her as the buggy space is now full. She asks the lady with the pram to get off the bus so she can travel with her friend. 'Pram lady' looks at her askance, and says sorry, but she needs to get home. Both pushchair ladies then proceed to loudly and verbally abuse the pram lady for being selfish and not getting off the bus, so they can travel together.
Everyone else on the bus was stunned into silence, the bus driver throws both pushchair ladies off the bus and drives off. Pushchair ladies stand at bus stop yelling and shaking fists at the receding bus!
I've never known so many bus passengers strike up conversation all at once, making sure the 'pram lady' was ok, and generally saying 'what a pair!'