There is this peculiar thing with humans and the attachements they build: some say that these attatchments are essential to their development. Now, Squall certainly isn't real, but we can certainly say he's somewhat human with some fantastical qualities.

When Squall loses his sister, he loses his innocence as well; thus, his struggle to become independent becomes more clearly visible. But to face such a hardship at such an young age disrupts his own development, stunting his own emotional growth. His childhood ends when he learns to cope with hardship, and becomes aware of his own emotional scars. Thus, Squall shuns away his genuine feelings and replaces them with more artificial ones; because he didn't deal with his emotions honestly, they eventually came back to wreck vengence.

Now, ask yourself "What would I do if I lost everything I had?". The two options would be to rebuild your life with what you have, or to never keep anything important so that you won't be able to loose it. Logically, the first option would seem the best, but certainly the hardest to obtain. For Squall, he chose the latter-- he would never allow himself to risk repeating the same mistake again. It would just be easier for him to deal with everything, even if he had to do it alone.

For a long time, this is where things remain. Regardless of his his physical and intellectual growth throughout the years, his disposition never changed; he continued to appear to be cold and heartless for the sake of his own defences. Squall believed that in order to survive throughly and efficiently all he had to do was rely on on himself; however, he only held this belief because he needed it, not because it was true. Throughtout the game, the most important people to Squall, whether or not he realizes this, constantly push him to rely on others rather than taking the burden full on by himself. In the deepest basin of his mind, the only way to heal his wounds was to be able to rely on someone whole-heartedly again.

However, before he could trust others, he had to trust himself again. In order to do that he needed some reassurance; he had to allow others to trust him. Supposedly, the first step was to make him commander of the mobile Balamb Garden. When that didn't work out too well, he was left to do with the body of a lifeless Rinoa. For once, he believed in his ability to help someone other than himself, knowing well that he was the only person who had the connections to do it. He picked up the pieces of a broken past and slowly rebuilt them; instead of painfully ignoring them, he would learn from them. Although his sister had abandoned him in his most desperate time of need, he refused to do the same to Rinoa.