Flirting

Teasing? Really?

I was with friends at uni the other day and a girl passed by and said hello to us (I know her). one of my male friends winked at the other guy (I'm less close with him but he is in the group of my friends) and smiled and repeated the whole act one more time. I asked what happened and they went she likes him. I asked how they could be sure and they said she tries to see him and talks to him. I asked the guy 'oh, really?' and the guy replied 'yes' with a smile. Then, I said maybe she comes more to see you, or maybe over a coffee or any excuse. Then he said stop to me that he is uncomfortable in an aggressive way. I caught off-guard due to the his attitude and I said I was just joking and he shouted don't joke to me again. (Actually I was trying to be sympathetic to the situation and happy for them; as we all do when friends shared good stuff such as going to the uni so saying u can get more opportunities; meeting a variety of people etc. what I really mean is kinda 'daydreaming' with them or being hopeful for positive together, I don't know the term and I couldn't defend myself there against hşm when he was extremely rude).

So, I wanna ask did I come off as teasing/making fun of him? I can't really get why he got angry.

Also, why is saying that someone likes our friend in a jovial tone considered as teasing for the desired person specifically? I mean how 'hey the guy fancies you' is offensive or humiliating for the directed person God's for sake? I really can't wrap my head around.

Moreover, I shared with my older sister and she told me it's playground stuff and in the future these kinda behavior might cause me troubles with people in different social settings. Unfortunately, I can't understand this much cuz she didn't elaborate. Can you guys elaborate for me as my third question? Thanks in advance for any answer.
Teasing? Really?
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