[Evisse the Blue]

To resume everything that was said and to add my humble contributon, I'll say, using a clinical analogy that there were:
a). a trigger factor (of the greatest importance): which is the Mt Doom event - IMO, not that Frodo couldn't let go of the Ring, but that it was forcefully taken from him when he was most attached to it
b).facilitating factors: the Weathertop wound, Shelob's sting, etc
c). vulnerability factors - that have to do with Frodo's personality. He was more prone to introspection, more reflective, and tended to brood more over stuff, then Merry - for instance, who, even though has himself been touched by the shadow, was too much of an optimist to let himself too absorbed by the consequences.

Therefore, all these combined - though a). is of greatest importance led to Frodo leaving to Valinor to seek healing and forgetfulness.