I have some questions about mesons. I don't really understand why they are build up from a quark-antiquark pair. I know from the theory that one can classify the mesons by considering the tensor product of the fundamental representation  and the representation [3'] (the prime for denoting anti-particles). But I'm wondering why you can't take the tensorproduct of two normal quarks, like x = +[3'] to classify the mesons. Or can't you have a bound state between 2 normal quarks?