'Pretentious' means, roughly, acting above one's station. Acting with pretense, to take the word back to its roots. "He's so pretentious" means, basically, "he's acting better than he is." And since the speaker is describing the speaker's own beliefs (who else's?), the phrase really means, "he's acting better than I believe he is."
That's interesting by itself. It means you can't improve somebody's opinion of you without acting pretentious. To get above your (old) self, you first have to figuratively get above yourself. After all, your old (perceived) self wouldn't be able to do what you're trying, so it's beyond them. And yet you're trying anyway. Pretentious much?
I started researching this post by going to Merriam-Webster and getting myself a dictionary definition of the word 'pretentious'. And it basically told me that I didn't need to write this. The English language pointed out this exact same thing for all to see. Now, I'll pass on that same nugget of wisdom to all of you:
1: characterized by pretension: as a: making usually unjustified or excessive claims (as of value or standing)
1b: expressive of affected, unwarranted, or exaggerated importance, worth, or stature
2: making demands on one's skill, ability, or means : ambitious <the pretentious daring of the Green Mountain Boys in crossing the lake — American Guide Series: Vermont>