Why do we have feeling of regret after having sex?

Both parties??: The majority of the time, i suspect a sense of regret might result from some guilt or shame issue & those are commonly derived from cultural or religious expectations or "upbringing". Treated as a "gift" given, in a show of affection, within the context of a caring relationship, it would seem unlikely to result in any regret! sometimes "sexualizing" a friendship may be the wrong move...