Any Zaydis here? I have a question regarding marriage.

When both shia and sunni are Muslims then why is not preferred for them to get married? I myself like a guy who is Shia and he also likes me. But he is a bit unsure about the future. Why are there sill people who say that both the sects can't get married. And why is it always portrayed like something which is not possible at all. The guy I like is a Shia Zaidi. He is quite serious right now but then again when it comes to the future, he is uncertain and not sure. He does not even give a chance or he is afraid to. First he was not even ready to fall in love with a sunni, now that he is he doesn't know about the future. I cannot seem to understand why. Is there even a chance?