B.F. Skinner called thoughts "mental behavior". He believed they could be rewarded and punished just like physical behavior, and that they increased or declined in frequency accordingly.
Sadly, psychology has not yet advanced to the point where we can give people electric shocks for thinking things, so the sort of rewards and punishments that reinforce thoughts must be purely internal reinforcement. A thought or intention that causes good feelings gets reinforced and prospers; one that causes bad feelings gets punished and dies out.
(Roko has already discussed this in Ugh Fields; so much as thinking about an unpleasant task is unpleasant; therefore most people do not think about unpleasant tasks and end up delaying them or avoiding them completely. If you haven't already read that post, it does a very good job of making reinforcement of thoughts make sense.)
A while back, D_Malik published a great big List Of Things One Could Do To Become Awesome. As David_Gerard replied, the list was itself a small feat of awesome. I expect a couple of people started on some of the more awesome-sounding entries, then gave up after a few minutes and never thought about it again. Why?
When I was younger, I used to come up with plans to become awesome in some unlikely way. Maybe I'd hear someone speaking Swahili, and I would think "I should learn Swahili," and then I would segue into daydreams of being with a group of friends, and someone would ask if any of us spoke any foreign languages, and I would say I was fluent in Swahili, and they would all react with shock and tell me I must be lying, and then a Kenyan person would wander by, and I'd have a conversation with them in Swahili, and they'd say that I was the first American they'd ever met who was really fluent in Swahili, and then all my friends would be awed and decide I was the best person ever, and...
...and the point is that the thought of learning Swahili is pleasant, in the same easy-to-visualize but useless way that an extra bedroom for Grandma is pleasant. And the intention to learn Swahili is also pleasant, because it will lead to all those pleasant things. And so, by reinforcement of mental behavior, I continue thinking about and intending to learn Swahili.
Now consider the behavior of studying Swahili. I've never done so, but I imagine it involves a lot of long nights hunched over books of Swahili grammar. Since I am not one of the lucky people who enjoys learning languages for their own sake, this will be an unpleasant task. And rewards will be few and far between: outside my fantasies, my friends don't just get together and ask what languages we know while random Kenyans are walking by.
In fact, it's even worse than this, because I don't exactly make the decision to study Swahili in aggregate, but only in the form of whether to study Swahili each time I get the chance. If I have the opportunity to study Swahili for an hour, this provides no clear reward - an hour's studying or not isn't going to make much difference to whether I can impress my friends by chatting with a Kenyan - but it will still be unpleasant to spend an hour of going over boring Swahili grammar. And time discounting makes me value my hour today much more than I value some hypothetical opportunity to impress people months down the line; Ainslie shows quite clearly I will always be better off postponing my study until later.
So the behavior of actually learning Swahili is thankless and unpleasant and very likely doesn't happen at all.
Thinking about studying Swahili is positively reinforced, actually studying Swahili is negatively reinforced. The natural and obvious result is that I intend to study Swahili, but don't.
The problem is that for some reason, some crazy people expect for the reinforcement of thoughts to correspond to the reinforcement of the object of those thoughts. Maybe it's that old idea of "preference": I have a preference for studying Swahili, so I should satisfy that preference, right? But there's nothing in my brain automatically connecting this node over here called "intend to study Swahili" to this node over here called "study Swahili"; any association between them has to be learned the hard way.
We can describe this hard way in terms of reinforcement learning: after intending to learn Swahili but not doing so, I feel stupid. This unpleasant feeling propagates back to its cause, the behavior of intending to learn Swahili, and negatively reinforces it. Later, when I start thinking it might be neat to learn Mongolian on a whim, this generalizes to behavior that has previously been negatively reinforced, so I avoid it (in anthropomorphic terms, I "expect" to fail at learning Mongolian and to feel stupid later, so I avoid doing so).
I didn't learn this the first time, and I doubt most other people do either. And it's a tough problem to call, because if you overdo the negative reinforcement, then you never try to do anything difficult ever again.
In any case, the lesson is that thoughts and intentions get reinforced separately from actions, and although you can eventually learn to connect intentions to actions, you should never take the connection for granted.