Aaron had always hated nicknames and the ridiculous endearments that people gave to their lovers. 'Hotch' was fine, a simple shortening of his family name, humanising him to his co-workers just enough, but keeping them at a bit of a distance, and people found it easy to remember. It also gave him the benefit of distancing himself from his father – the original Mr. Hotchner – but without seeming suspicious or unusual. Hotch didn't want anything about him to be like his father.
Couples' endearments held bad memories for him, constantly being used sneeringly and sarcastically by Haley during heated arguments (mostly from her side, Aaron hated arguing) they would have, seemingly over everything Aaron did, no matter how hard he tried. After too many "Yes dear"s and shouted "no, of course it's fine to leave again, sweetie"s when cases came up during off times, all sweetness or love seemed to be lost from sweet nothings.
So he himself was very surprised when, during a night of snuggling on the couch after Jack had gone to bed, Spencer had sleepily uttered "love you, darling" and it hadn't made Aaron think of an argument or cutting remark. Evidently, his lover had been slightly surprised too, suddenly fully awake, shifting to look at the older man's face with a hint of embarrassment on his own. He opened his mouth to explain himself, knowing (but not knowing why) that his lover never used endearments, but was cut off by the wide smile on Aaron's face which got closer to his when the older man bent down to give him a chaste kiss.
'Darling', it was old fashioned, un-cheesy. So Spencer, so them, that Aaron couldn't help the smile, the kiss. So them that Aaron couldn't help the "I love you too, sweetheart".