Reasoning often suffers from a bad press (and not only in high school where nerds are unpopular). It's supposed to be tiring, boring. We would be better off following our gut instincts. And sometimes it is true. But I would like to claim that this is not due to reasoning as such, but to the fact that it is applied to domains that often don't seem relevant to us. Let me use an analogy here. Using our motor control mechanisms can be extremely fun: having a greater control of our own body is one of the pleasures offered by sports generally. But using motor control can also be mind numbing if you work in a factory, or simply if you don't like sports. The difference is that in one case you are applying your skills to something that is relevant to you (being good at squash if you like squash) and in the other to something that is not so relevant (being good at stomping pieces of metal or playing squash if you don't like squash).

It is the same for reasoning: if you like philosophy, or math, you will enjoy reasoning about these topics, if you don't, then you won't. OK, this looks like a truism. My point is only that reasoning is not intrinsically boring, and I hope that if you didn't agree with that from the start you will now. Actually it would be surprising that any cognitive activity would generally have a negative hedonistic value, at least if you are an adaptationist. Using our cognitive mechanisms has to be good for us (otherwise we wouldn't have them), and the trick that natural selection usually employs when it wants to make us do stuff that is good for us is to make doing this stuff feel good (sex being the perfect example). So if reasoning is an adaptation, then we should feel good when we reason. And guess what: we do.

This is obviously not always true. There are several components that determine when reasoning will feel good. One is the relevance of its result (cf. the sport analogy). If, in any given domain, you want to make sure that the claims you make or you read are well justified (have good reasons supporting them) then reasoning about these claims might feel good. Another factor is the fit of the input with the requirements of our reasoning mechanisms. Some problems will be easier to reason about because they have a well defined form, while others are very fuzzy and make our head hurt.

The second point is, I think, what explains the success of Sudoku. To find the answers of a Sudoku grid, you have to reason: you have to compare different hypothesis (is it a 4 or 7 here?) and find which has the good justification (it can't be a 7 because it wouldn't fit with this other line). Sudoku is an artifact that has been made to tap into our reasoning mechanisms. And as other artifacts (such as make up), it might create stimuli that are actually better at taping into some cognitive mechanisms than the natural stimuli that these mechanisms evolved to process. In the case of reasoning, it is not clear what these natural stimuli are (I wouldn't claim to have made a convincing case yet), they clearly are not Sudoku problems (unless we find some grids in a new Lascaux).

And I think that both points explain why some people are fond of doing math or philosophy: they find these domains relevant and in these fields people have been creating things that are good at tapping our reasoning mechanisms, be it a mathematical proof or a philosophical argument. So reasoning in this case might feel really good: first because the result of the process is relevant; second because the inputs are right and the right input in the right place makes us feel good (see this post).

Spinoza is probably the epitome of philosophers who tried to built their entire system on pure reason. One of the reasons for this might be the strength of the hedonistic value reasoning had for him:

I finally resolved to inquire whether there might be some real good having power to communicate itself, which would affect the mind singly, to the exclusion of all else; whether, in fact, there might be anything of which the discovery and attainment would enable me to enjoy continuous, supreme, and unending happiness. (The Treatise on the Emendation of the Understanding)

In her book on Spinoza's life (Betraying Spinoza), Rebecca Goldstein (1) describes this as "ecstatic rationalism". So reasoning can be more than fun: it can lead to a sort of experience that one would more easily expect of a Buddhist monk than of a rationalist philosopher.

From now on, when you try to fill a Sudoku grid while in the subway, you can think about your feeling as a glimpse of what somebody like Spinoza felt like when he was trying to understand the deepest secrets of the universe.

If you want to know more about the The Psychology of Sudoku Problems.

(1) People: Rebecca Goldstein is the new girlfriend of Steven Pinker.