Even if you want the thing, some of the stuff you need to do to get the thing may be less immediately rewarding than other stuff you could be doing instead. Even if you consciously know it's worth it in the end, there are subconscious forces within us that are not as far seeing.

So you need willpower to get past some local lows. Without it, you'd have to restrict yourself to goals that can be accomplished with only constant enjoyment along the way. That leaves out many worthy goals, and most opportunities to create value.