If you reject him at ANY point during the relationship (even at the most cruel places, like right before the Omega 4 relay and on your bottle shooting date), he never once gets angry (embarrassed and hurt, but he never really has a super angry rejection). He never ONCE is all “BUT SHEPARD WE’VE BEEN FRIENDS FOR SO LONG Y U DO THIS”. He’s cool, understands her reasons, and NEVER feels entitled to her, despite how much he may love her. Like A++ act right there ok.
And then he’s totes 100% cool with you inviting your ex back on board (if you romanced Kaidan or Liara and broke it off), and is even happy to see them because HE WAS BROS with them too. There’s no jealous bullshit because he respects Shepard and TRUSTS her.
Seriously Garrus should be a role model to men everywhere on not being a douche. Just saying.