Why would a guy friend get very nosy about your personal life?

He keeps asking me questions about my other guy friend all the time with whom I'm much more closer than him. Whenever I talk about this guy friend of mine he clearly displays jealousy and then asks a lot of questions about him i. e. invading my personal life.

This week my other guy friend attended Math class with me and he was sitting with me so my guy friend started acting mean with him for no reason and was even chuckling when he gave the wrong answer.

He's really creeping me out now. This week, our Math teacher was giving me an awful lot of individual attention (he acts very obsessive with me too) so my guy friend got jealous and quickly said "Woah, you've got all the teachers wrapped around your finger haven't you?". What he meant by all is that our English teacher gives me an awful lot of attention too.

Yesterday around 3 am, I was missing my first love a lot so I posted this quote about how love fucks you up real bad and he instantly responded to it within minutes saying "Oh is that so?" with sarcasm. This is not the first time he's viewed my updates at odd hours.
What's up with him? Why is he suddenly acting all clingy and nosy? Should I be worried or not?
Why would a guy friend get very nosy about your personal life?
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