I have a boyfriend so why am I feel so sad that my guy friend is leaving?

We met each other in office like 4 months ago and we became friends (got in the same friends circle) and he's been attracted to me from the start, it was him who initiated the friendship, he keeps staring at me all the time when I'm not looking quickly looks away when I catch him, sometimes he doesn't care that I'm staring, he has tried to get intimate with me once too, flirts with me a lot and gets jealous of my other guy friends.

Anyways I started ignoring him slowly since last month because he has a girlfriend and I didn't want to get involved in all this. I don't know if it's coincidence or not but shortly after that he's become totally uninterested in work and he was very passionate about it, he doesn't seem like himself anymore, mostly comes late to office and leaves early the reason being he keeps drinking during those hours (sometimes he even comes drunk to office and sits nexts to me).

I found out yesterday that he's planning to leave after this month and since I heard this news I feel very sad. I don't know why. What's going on?
1 mo
He seemed very hurt and upset that I have stopped talking to him. He has even stopped talking to our mutual friend who hangs out with me the most and sits me in the office too. Else they were very good friends before and used to hangout all the time.
I have a boyfriend so why am I feel so sad that my guy friend is leaving?
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