That says a lot right there. Others are a priority but his significant other, he can take for granted.

There could be many reasons as to the 'why'. I don't think the 'why' matters as much as his actions showing 'you don't matter as much to me'. Does it really matter why he does not give you what he gives to others? The fact is, he is *not* giving it to you.

How would it help you to know his 'why' (he might not even tell you or lie to you - so, how would you even get to the truth of his 'why')? Would you actions be any different if you were to know why?

Sometimes (often?) in life we just can't know the why of what is in other people's heads. Their actions are a good enough pointer to how they operate. This is reliable data i.e. their action vs. this unknowable 'why' in their heads.

Regardless of the 'whys', he is treating you poorly. Not feeling good in a relationship shows it is *not* a good relationship.