Why did my crush got angry when I walked away?

i asked a guy about his relationship status, he said he was seeing someone casually for a while. He asked us to be friends. i said oki.
After a few weeks i told him P. S I LIKE YOU, he replied back saying he has a GIRLFRIEND and she is upset about us talking. i was taken aback. i apologies.
After a few weeks i asked him again if he was not interested to go out with me and be lovey-dovey. He replied that i was not respecting his relationship and his choice.

I was stuck since i had strong feelings for him but was not able to expressed them so i talked about walking away and he did not replied.

when i did he got angry.. why?
Why did my crush got angry when I walked away?
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