Before I write more about monotropism -- or the mind in general -- I want to talk about the word 'experience'.
I think that people often confuse two very different concepts: To talk rationally about which life pursuits are (or not) important is one thing, but to experience motivation is something else. Experiencing motivation (or experiencing interest or fear) is largely a neurochemical reaction in which arousal levels are either heightened or subdued.
Depression is, on one level, a disorder of motivation. I am using the word monotropism to refer to a very different sort of disorder of motivation: one that is less mood-dependent, but in some ways more pervasive. It's like being a website with few pages. Most things in life are experienced simply as links leading off of or onto one's pages.
I believe it's possible for a person to simply lack the hardwiring or neurochemicals that are needed in order to experience motivation normally -- by that, I mean that there's a lack of intrinsic/instinctual motivation toward very basic things like comfort, security, or cleanliness. A monotropic person may experience, at the neurochemical level, very little motivation by anything that doesn't relate to their particular passion (their cause or purpose, their relationship, their career...)