From this interview with the costume designer:
I do like his helmet better this time around too. It’s more aggressive. The horns in the first helmet went straight up and curled around to the back. This time around, I wanted to treat it more like something that came forward towards you first, then tilted back. They’re a bit longer and thinner, which is something to make him look more mischievous and more of who his character is. I felt like I got to play more into what his character is to be shown on his costume.
A lot of that comes from the gesture of his horns, the curve that comes outwards towards you but curves back towards him. That gesture is pretty aggressive at the beginning, then curves down towards his spine. It does more of a whiplash thing towards the audience or whoever he’s looking at. It’s already a type of in-your-face thing. For me, that was the biggest part of his initial read, to make sure that gesture came across.
Basically, the horns were designed to reveal something of his character, his personality where he's relatively brash and in-your-face, but much of who he is, and what he does, is hidden further back.
Alternately, if you're looking for an in-universe reasoning, it may be a hint to Loki's origins, as the Marvel depictions of Frost Giants often had them with horns on their helmets or on their heads. This fellow certainly posits something similar, also tying it in to a concept of "sorceror's horns" (I haven't found any links directly on that concept, but there are discussions of various pagan groups dressing in imitation of their "Horned God" by wearing hats or helmets with mounted horns). This would actually meld well with Odin also having similar (but smaller) horns, signifying that Loki is primarily about the magic (large horns), Thor relies on physical force (no horns), and Odin seems to use the two interchangeably (smaller horns).
For what it's worth, Tom Hiddleston claims that the horns have to do with a tie with the Christian depiction of Satan having horns (which is known to be a result of them literally demonizing the pagan horned gods).
"You've got two big horns and... the imagery is satanic," he said. "People who don't know the comics as well, I've shown them the pictures . . . and they say, 'You're playing the devil. Your'e playing an incarnation of the devil.' And he is the Marvel agent of chaos."