When shou and ritsu decide to try out a RelationshipTM, shou is very, very insecure. Because, well, what if ritsu likes him today, but wont anymore tomorrow? Or the day after that? What if he never stops liking him back?
Afterall, he knows that relationships only really go two ways: either you break up eventually, or youre together forever.
He doesnt really know which one terrifies him more.