Why would you smile at a girl but not approach?

So a guy in the bus caught me looking at him and then I looked away.

I looked back and he smiled as he was walking down the stairs to leave the bus.

I kinda inside panicked and didn't smile back but kept looking at him. Then I run into him again on the street 10 mins later... as he was singing phantom of the opera (he turned out to be a busker lmao)

As soon as he saw me he started singing to me so this time I made sure I smiled back. Anyway, he kept looking at me but didn't approach. Why would he try to smile at me and do all this but not try to approach?

I don't get guys

Why would you smile at a girl but not approach?
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