I'm a feeder, not a feedee, but this is a subject that fascinates me and which I've studied informally for years.
There are two kinds of feedees: those who have thought about gaining since childhood or early adolescence (let's call them "died in the wool" ) and those who discover the appeal of it only after gaining "accidentally" or to please a feeder, or because they stumble on it on the internet and realize they like they idea ("late bloomers" ).
The reasons they start when they do are usually one or more of the following:
1. Many feedees of both categories start when they have an "accidental" or "opportunity" gain. This usually happens when their situation changes and they sudden have an opportunity to eat more fattening food and/or larger quantities of food than they did in the past. This typically happens when they leave home and/or get a job that gives them the freedom to eat what they want. They start overeating for the fun of it. Another cause, of course, is pregnancy. They gain without really planning to and then they realize they like gaining, or that they like it even more than they thought they would and continue gaining.
2. Some start gaining when they meet a feeder who convinces or coaxes them to gain. Some died-in-the-wool feedees don't want to start until they find the right feeder.
3. Some start when they find the weight-gain sites on the internet (e.g. this site) and realize they're not alone. Some of these start almost immediately upon finding the sites; others are gradually convinced to gain after visiting the sites for awhile. Some don't start on purpose but find after visiting the sites for awhile that they've gained weight and like it.
If you're actually looking for advice on when or whether to start, see my article "Deciding to Get Fat" at
askdrfeeder.dyndns.org.