He was dwelling over his past and his losses in AC, but that's understandable - if you consider everything he went through. From the destruction of Nibelheim to Hojo's experiments, from discovering and shattering his own fabricated SOLDIER identity to failing to protect Aerith, he had to endure degrees of pain and loss that most people could never compare to. And while he was going through all that, he had to lead a tiny group of misfits against a god-like enemy, with the fate of the entire world hanging in the balance.

Once that was all over, it's small wonder he turned inward and struggled to come to terms with it all.