People will sometimes "bump" a thread to keep it active, or at the top of the forum list, so people will see it and will be more likely to check the thread out. If this was allowed, then people would spam up their threads with garbage just to keep it at the top of the list for others to see.

The natural lifecycle of a thread (if there is such a thing) tends to be that the thread is created and gets some activity, then after a while it drops in the list of threads (sorted by last post date/time) until it drops low enough that it gets pruned because it is an inactive thread. It's nature's way of keeping the forums clean and up to speed, kind of like nature, where the weakest/oldest animals dies off, or in this case the weakest/oldest threads die off.