OK; from a man who likes other men, when it comes to sex BETWEEN men, the anus is the only opening you can penetrate. So all the physical arousal and emotional intimacy that's centered on the vagina in heterosexual intercourse is centered instead on the anus.
For straight men (I am bi for clarification) a preference for anal usually is either due to physical aspects or psychological ones.
Physically, if your member is on the smaller side, or if you are used to a vigorous masturbation style, the anus will be a tighter fit and a rougher texture, so provide more satisfying friction / stimulation.
(If you are receiving, your prostate gland will get nudged, which is extremely sexually satisfying for many men.)
Psychologically, a lot of men have a kink for some form of transgression. So simply the fact that going up the butt feels more transgressive and 'naughty' to some, would be the prime turn-on.
For a heterosexual couple it is also a huge gesture of trust / attraction from the woman. SOME do genuinely enjoy anal intercourse (those who do usually describe it as a kind of secondary stimulation of the vaginal canal since only a couple layers of tissue separate it from the rectum), but a majority seems not to like it all that much. So merely having a girlfriend or wife 'let you go there' would be a big deal for a lot of men and a turn-on in itself.
... I hope I did my best to explain.