That, and there's a big difference between the connection two people share after having been involved" with one another for a number of years and the feelings one has for another when first meeting them. When you first start dating someone, you want to spend practically all of your free time with them, which explains why Aeris' "love level" was higher than Tifa's at the start. Cloud had a more meaningful connection with Tifa because he had known her a lot longer. Aeris ws the new girl and therefore, the short-term connection was stronger.