We'll be talking or something and he'll randomly say, "You're cute" and because it's so out of the blue, I ask why he said it and 9.8/10 he'll never have an answer.
For the record this is a longtime boyfriend and only reason I'm questioning is because sometimes he'll say it when I've gotten dressed up sort of(put on make up, wore a nicer top than usual), and it'd be nice to hear 'pretty' or 'beautiful', which are two words that are rare and it's not that I need them all the time, I just don't always want to be cute and feeling like a little kid...
If he's calling you that way then he probably have a crush on you. I don't usually say to a girl if she's cute unless she ask me "how do I look?" that's when I tell her and if she's ugly as f*** then I would probably make up an excuse of not telling her how she looks, thankfully I've never had ugly girls in my life and never dealt with them and I hope I won't cause I'd probably choke myself if I'd get that question but yeah.. if she's telling you're cute then he's probably into you :)
It's his way of showing verbal affection. It's easier to say than, "I love you". As for his choice of the word "cute", maybe that's his preferred word for someone who looks nice. Mine for my last girlfriend was "hot". Buy him a thesaurus for his birthday or Christmas and bookmark synonyms for "cute". It's a nicer way of taking a jab at his lack of vocabulary.