Don't allow yourself to get even mildly infatuated with someone you are never going to see again.
It is normal to regret missed opportunities, but put a leash on what you consider "opportunities". These are not proper opportunities.
When going about their business, people are rarely open to anything other than phatic ("hello, nice weather isn't it") or basic transactional ("can you move down inside the carriage please") interaction with strangers. That's a fact. In London, people even hate phatic talk.
However - get a hobby that allows you to meet women, behave appropriately, and you'll be onto something.