Let's start at the very beginning, with what we know about the characters and their personalities.

Cloud is (at the start of the game) quite cold, impersonal, uncomfortable with showing he cares about anything. Even so, he left home to join SOLDIER to impress Tifa, because he's crazy about his childhood friend.

Tifa cares about Cloud an awful lot, but is not entirely sure he is who he says he is. Between that and the fact that he doesn't let himself act like he cares for her as anything more than a childhood friend, Tifa's not sure how he might feel, and so she does not express her feelings for him.

Then, along comes Aeris. She obviously finds Cloud fascinating, and unlike Tifa, is not afraid to show it in a little flirting and just general friendliness.

Cloud is amazed - he's never had anyone act like this towards him before. Especially not Tifa, who he's had deep feelings for since childhood. But in all that time, she's never shown him anything that would make him think she likes him as more than a friend. Therefore, when Aeris flirts and acts playful, he shyly tries to reciprocate. After all, it's not as if Tifa cares, he thinks.

Tifa can't complain - she always thought he didn't care about her anyways. More reason not to tell him how she feels. Let the two of them have their fun. And at least Aeris is a nice girl...

Time passes, and Cloud and Aeris grow closer. With her encouragement, he begins to open up a little more about his feelings, and he realizes he's fascinated by her in a completely different way than he was with Tifa. She's so open... Tifa never acted this way, he thinks. She couldn't possibly love him. But Aeris isn't bad either... and Cait Sith says they're perfectly compatible, besides.

Then it happens - Aeris is murdered. Soon after, Cloud discovers he's not who he thought he was. Mentally, he's destroyed, he's lost everything he had to hold onto...

Except his feelings for Tifa.

In the lifestream, he lets her see, for the first time, how much he really liked her when they were growing up. He never would have let himself show these hidden feelings, except that Aeris helped him learn how to open up to people. She showed him that feelings weren't anything to be ashamed of. So now that he's learned that lesson, he can share all the things with Tifa he never thought he should.

So that's my little dissertation on the FFVII love triangle. Just my opinion, of course. I do think Tifa and Cloud were meant for each other, but I also believe that his short-lived relationship with Aeris was necessary, so that he could learn to open up to Tifa later on. So... there you have it.