I can understand from what thoughts this opinion may derive, but it only shows that you've never talked with someone who is into BDSM.
For me submitting to my partner is an act of great affection. It takes alot of trust and willpower to be able to submit to someone, give in to him and let him do with you whatever he likes to do. I couldn't do that with a stranger, I need to be close to my dom/seme and trust him/her alot. Also my sexual relationship is happy, fun and feels good for me, but it wouldn't if I was not to sumbit to my partner. If I can submit to my partner and give him/her control over me I show him just how much I love him and how much I am able to trust him.
Concerning pain and torture, well that's a fetish thing. You don't have to feel weird if pain doesn't arouse you ;D
In my case pain can give me alot of pleasure, but not every pain. I won't get a boner when visiting the dentist xD Almost any pain outside of the bedroom feels bad like it does for any other person. But if it's at the right place and the right area where I recieve pain, it gives me great pleasure. I can't control that, it's kinda like you can't control if you're gay or not ^__^
So yeah for me inflicting pain to me is a way of showing affection. This also needs alot of trust for I know myself how hard it can be for one to hit somoone else. I tried two or three times to be a top and dominate someone and I just couldn't do it. It's really hard for me just to slap someone even though I know myself how good it can feel. Given that, it is sign of great affection if someone can overcome himself and spank the shit out of me :3