Antonym of "phobia"

If a phobia is to have an irrational fear of something, what is the word for having an irrational affinity for something?

For example a numerologist may fear the number 13, but be attracted to (or even have a love for) the number 8. (Some would say this is irrational.)

As @FumbleFingers points out, the -phil- suffixes are the closest opposites you'll find:

Suffixes with the common part -phil- (-phile, -philia, -philic) are used to specify some kind of attraction or affinity to something, in particular the love or obsession with something. They are antonymic to suffixes -phob-.

However, in many cases, we tend to think of passions as a positive thing, so I'm not sure these are the antonyms you're looking for. For example, if I describe myself as a technophile, you might think that I am forward-thinking (and it would be the technophobe who might be condemned); if I'm an oenophile, I'm merely sophisticated; if I'm a bibliophile, just educated.

As an alternative, you might consider the -mania suffix, which "denotes an obsession with something". This would more closely fit the meaning you're looking for of an irrational attraction.

For example, bibliophilia is sometimes contrasted with bibliomania, where the former is "the usual love of books" but the latter is

a symptom of obsessive–compulsive disorder which involves the collecting or even hoarding of books to the point where social relations or health are damaged.

Although courage and bravery are the perfect antonyms to fear, it seems you wish to capture the irrational nature of a phobia, for which I think there is no perfect antonym. If you want to be specific, I think you should say explicitly that the affinity is irrational. Otherwise desire, lust or crave may do.

I think this is a loaded question, as it contains a controversial assumption. I don't agree with what you say that an attraction or love is the opposite of phobia.

As you yourself properly noted, phobia is a fear of something. The opposite of being afraid of something is being brave towards it, which is defined as bravery or courageousness, which in my opinion both are proper antonyms of phobia.

Now whether courage is rational or not is very difficult to distinguish and it raises a few philosophical questions, which aren't fit for this place, but all in all it's safe to assume that if the following is true:

The man who entered the building on fire and rescued the unconscious woman must have been extremely brave!

Then bravery is irrational, as going toward fire (danger) is not rational at all. It can be considered stupid, irrational decision and if one did that trying to save someone, but dying instead, their actions could easily be considered stupid, irrational.

Therefore I believe it's safe to say that an antonym of phobia is courage.