If you are a parent who travels by bus, you'll know it's sometimes not possible to get on if there are already two prams.
I was waiting patiently for my bus - with my daughter in her pram. Another mum with a pram came along and when the bus arrived she got on first - and I couldn't get on.
Am I being unreasonable to have expected her to have allowed me to get on as I was there first. (Maybe just naive, not unreasonable!) (For the record, I always allow whoever was there first to get on before me).