Terra has one dream and he's trained his whole life for it. He wants nothing more than to be a keyblade master. He shares this dream with his closest friends Aqua and Ventus. Together, they train, inspire, and encourage each other to keep moving forward. This has created a deep and unbreakable bond in the three and helps push them forward while keeping them on task.
I consider the relationship of the three keyblade wielders to be as siblings. Terra strikes me as an older brother not only to Ven but to Aqua too. I feel like he's probably spent his whole life training to be strong enough to look after everyone. I think this has a lot to do with being raised by a keyblade master. Terra's respect for his father (figure) Eraqus lends a lot to the way he behaves. His respect is so deep, one day he hopes to walk along side of him as a master. I feel like this desire to protect his home and his family is what drives him the most. He wants to share the goal but at the end of the day, I whole heartedly believe Terra would tear down the moon, sun, and stars to keep his family safe.
The closeness of family also lends to how well and easily he trusts. In Terra's case much too easily. Terra trusts too much, in fact, and it causes him much heartache and confliction. Terra believes in doing the right thing yet does not always know how to approach topics, subjects, or issues without internalizing the situation. Although he acts on instincts he is often drawn to causes and battles for his pride as he tries to gain understanding of himself and his abilities. I submit, also, that this bond of family is ultimately his saving grace. He trusts too much and he loves very deeply.