Quickbeam may as well mean "Quick support-of-the-home." Quickbeam's home was destroyed; he was a victim. He quickly sought revenge and jumped to the conclusion that he must fight on the right side and protect the forest (home) he had left.
Quickbeam never wasted time in thought or "took long drinks," like the older men (meaning he did not drown in sorrow first). He drank quickly and grew quickly past his troubles and set out to do something about it. It's safe for Quickbeam to jump to conclusions because he jumps to the right conclusion; he doesn't need to think much. Treebeard, on the other hand, needs to strategize for a long time and mull it over. His temper is as short as Quickbeam's, but his temper is destructive, so he must drink and sleep on it first before he does anything. It's easy to like them both in different ways.
