Great thread, silvercamel. I think the basic answer common to most if not all bottoms is they either like or don't mind getting fucked. Beyond that, I think it depends on each person's own circumstances that flesh out the answer. Peterh described his; I will attempt to do so for myself without writing a treatise on my bottomming history. But history does play a part for me.
I was married to a woman for 22 yrs and had 3 kids; so I've topped. And to be truthful, when I came it felt good, but other than that I could have cared less. As I began and during my 14 yr journey out of the closet (I came out in 1988), most of my sexual partners were either straight or bi guys who were either hustlers, street trade, or from an escort service. From '74 to '83 I was in the DC area, and most of my contacts with these guys were in xxx bookstores or movies. And most of those encounters were oral. Whenever I had to stay overnight in DC or travelled, I would have a hotel room, and could take the person there. Those were the times when you could really explore various positions and role playing techniques. I took to bottomming like white on rice. Then when I moved to Pittsburgh in 1983, I was staying in a hotel until my family arrived in a month and it was then that I encountered someone who would change my whole bottomming experience forever.
He was from an escort service and was a medical student. He was the one who showed me the joys of prostate stimultion. He showed me with his finger first and his dick was long enough to hit it every time he thrust inside me. While I had evidently experienced some of this pleasure before, I never felt it as intensely as I did that night. And from then on, anyone who topped me had to perform that way.
I think it also depends how one defines top, bottom, versatile. In the old days we would use the terminology: French Active (meaning you give blow jobs but don't get them); French Passive (you get blow jobs, but don't give them); Greek Active (you fuck but don't get fucked); Greek Passive (you get fucked, but don't fuck). So you would see ads in the local gay papers for escorts which would ue some variation of those terms, depending on what the person did or didn't do. I'm sort of a strict constructionist based on my past experiences and preferences. For me a bottom is some one who gives blow jobs to his top and gets fucked by his top. Some consider bottomming from a differing viewpoint, i.e. if you get fucked, whether or not you fuck others, you're a bottom. And I think that's where the term Versatile comes in. To me that means you will do oral and anal, active or passive. And that's just the physical part of bottomming.
There also can be an emotional side. For the suck and fuck encounters, I just wanted to get fucked and that was it. But in a few instances if I would hook up with the same hustler frequently, then an emotional side arose. There was more than just the money. I got to know the person over time and there was a bit of a connection there. In two cases, there were guys I met as hustlers but our friendship extended long after they put hustling behind them. Therre was a real connection in each instance and for me there was nothing more satisfying than sucking their dicks, getting them nice and hard and having them fuck me until they came inside me. Their physical pleasure was an emotionally rewarding experience for me. And then they would pleasure me by fingering me and massaging my prosatte while we were kissing and I was jacking off. My climaxes were always intense and again, the look of satisfaction on their faces that they were able to satisfy me in the way that I liked was an emotional high for me as well. (These were separate encounters; they were friends and this situation arose out of a unique set of circumstances. We never did a three way amongst ourselves.)
As to the pain, again each person hss a different level of pain tolerance. I have a high level of pain tolerance which is good because I am a size queen and I have had for the vast majority of my encounters larger dicks. When I first started bottomming there was the pain of the initial breach, but after that, not too much. Then as time went one, I was taught the importance as well as joy of rimming and fingering. It helps to relax those sphincter muscles so the breach is not as painful. But a little bit of pain can be very erotic depending on the circumstances.
As to positions, again it depends on personal preference and the mood I'm in. I am submissive in bed. Depending on the mood I'm in I can be mildly submissive or really want to be dominated. I love doggie especially if I'm in a more submissive mood. But it's up to my top. When I'm with certain people, I want to be more romantic, and no matter how we start out, I like to end with missionary.
I didn't mean to ramble, but it seems I did. But I think there's no easy answer to bottomming because it depends on each person and his needs and desires. I hope this will be helpful to you in understanding the topic of bottomming.
-