He has been in a relationship for the last 6 years. Yet I'm the first person he calls after he returns from his trips and not his girlfriend. Why?

He even gets pissed if I don't contact him often or go silent on him. Why?
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He used to like me before he started dating his girlfriend. But i wasn't ready for a relationship back then. But after being in a relationship for last 6 years why he is like this with me.
He has been in a relationship for the last 6 years. Yet I'm the first person he calls after he returns from his trips and not his girlfriend. Why?
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