Head cannon - boss thinks sans is super cute and wants everyone to know that his sans is the cutest ever , as well as having others telling sans he's cute/awesome without having to himself. Its a back handed complement/brag
I hate when people give me a back hand complement being like “ OMG your match at Takeover was so good. You’ve really gotten so much better.” Hmmmmm no! I’ve always been this good, you’ve just been to blind to see it! #BOSS 😎🙅☺️.