Also, while Cloud is not in the persona of his true self at the beginning of the game (but rather a mix of Zack and Tifa's memories of him), he still is, you know, self-aware as a human being and continues to build upon his experiences and gain memories. This is evident in the fact that as the game goes on, even way before the lifestream scene, Cloud becomes less and less the cocky mercenary jerk (Zack) he had been when he had just awakened form Mako poisoning. This is why we do not notice a stark contrast between Cloud's personality at the beginning of disk two and his personality at the end of disk two; He had gradually become more and more like the real Cloud on his own, with the only exception being his frequent mental breakdowns. This is why Aeris is eventually able to sense the 'real Cloud within' on the Aeris date ("No, but you're different") and differentiate him from Zack. So when he remembers his motivations of the past (wanting to join SOLDIER to impress Tifa), it doesn't mean that he automatically throws away all the more recent (and in my opinion, more relevent and timely) motivations and memories which compells him in the present (Save planet, defeat Sephiroth) even though they were developed in a time where he was not completely himself on the surface. Also, this is why his earlier crush on Tifa may not have necessarily transferred over to the present as affection.