-- at least the kind who do it for money. Setting aside for a moment the moral issues, it comes down to risk and reward.
On the risk side, if you are caught you can expect to die in prison, whether by execution or by simply never being released. There is also the real prospect of torture, a prospect not made more pleasant by the euphemisms (such as "enhanced interrogation") that may be used for it. These are some large risks, folks.
On the rewards side, I can't help notice that most people (like, for instance, the Walker family) who spy for money seem to do it for a tiny amount not at all in proportion to the risks they take, not least because after even the first instance, the spy is already vulnerable to the above-mentioned risks. In most cases that have come to light, the spies seem to be paid a few thousand dollars per job. Wouldn't it be safer to steal a car or burglarize a house for jewels?
On the other hand, if a spy were to make an extravagant demand for money, wouldn't the spymaster simply find it easier to kill him at the end rather than pay him? The whole business was already secret and illegal, so as long as the spy's death did not generate too many questions that might eventually lead back to the spymaster, there would not seem to be much obstacle to this.
The upshot of all this is that it seems unlikely that a meaningful percentage of spies really are motivated by the money; like a gambling addict, the money is a secondary consideration after the thrill of taking an insane risk. Presumably such things are taken into account when security clearances are granted; someone who enjoys free-climbing rock faces, for example, may require a second look.
So my conclusion is that "normal" people don't become spies. This seems to be confirmed by the descriptions of double agents in Ben Macintyre's book Double Cross: The True Story of the D-Day Spies, which I started listening to as an audio book on my drive back from visiting my dad for Thanksgiving. To be fair, the double agents described in the book opposed the Nazis on principle, but they were still an odd group.
To the best of my knowledge, no country makes much distinction between a foreigner loyal to his country who spies on them and a citizen who betrays his homeland, but it seems there must be both a moral and a psychological difference between the two. Macintyre's book mentions other German agents -- at least some of whom were also in fact Germans -- who were captured in the UK but refused to turn double agent; they were sent to prison or executed. In fact, although all the German spies in the UK were apprehended, only a small percentage of them were turned, since some honestly refused and others were obviously untrustworthy. Were the Germans who refused to turn less ... eccentric than the high-profile double agents that are the main subject of the book? Probably; which is not to say that they would have been exactly ordinary people, though.