The Tide of Changing Worlds
Thanos is a shardmind that believes in the god shard philosophy. Basically they believe that they can rebuild the living gate guarding this realm by becoming personally powerful. So they seek challenges for themselves so they become stronger.
Thanos himself is fairly newly formed, only a few years old, and does not truly understand what motivates these organic beings around him. He tends to hang back and watch those around him trying to learn from the interactions that he sees.