he is not prepared to take things any further.
He doesn't get the reaction he expects when he tries to impress you.
or maybe he is an A...i have heard guys say over and over again..."she's too serious" about me and about other girls...."too complicated" is the new "too serious"...forget about it unless you hear it again from someone else....do you have any friend or family that you know will be honest? Ask them...since they know you
My guess is that he means one of two things. Either he can't figure out what you want from him because you give off mixed signals so he's unsure whether you're really into him or not... or he means that you have a lot of emotional baggage that causes all sorts of drama. (For example -- crying spells, needing a lot of reassurance that he cares about you, being overly jealous and possessive, etc. are all things that would be a huge turnoff to most guys.)
Probably that he thinks you're too complicated.
Maybe he thinks you fuss about one aspect of your life disproportionately to others. Maybe he just doesn't want a long-term relationship right now but doesn't want to admit he has commitment issues.
Because he's holding off for something better. If he can't find something in the immediate future, he wants to have you around just in case. Of course he won't say that outright to your face.
I don't know you from Eve, but you're not unattractive. Why is he considering new girls if he's already in a relationship? That's like using company time and resources to look for a new job. It's a good way to get fired and a sign of a scumbag.
If you are in a relationship, either he is looking to end the relationship (and there a could be a million reason for that but fear of commitment probably ranks highest) and this is his cowardly way of shifting the reason for it ending to you; or he is just too lazy to spend the time to get know, understand and appreciate you.
Contrary to popular belief many guys are not the lust-obsessed unemotional machines we are made out to be; although I would suspect as female you will have been bombarded with tedious obscene messages by enough guys here and elsewhere to find that hard to believe. However most guys are lazy when it comes to relationships and understanding women, so I would tell him to stop being lazy and put some effort into it!
Sorry a bit of a wordy answer, but hope it helps.