Still don't know why...??

[TheAbominatrix]

In VII, every choice you make effect your 'affection level' with the other characters. Namely Aerith, Tifa, Yuffie, and Barret. For example.

Buy flower +1 to Aerith
Give to Tifa +1 to Tifa
Give to Marlene +1 Barret

These affection levels affect a few things, mainly who you go on the (SPOILER)date at Gold Saucer with and how much affection Tifa gives Cloud (SPOILER)when they're alone near the end

Edit: I'm trying to dig up an FAQ on this, where it shows what choices to do what to whose affection level.