Any activity you engage in that cuts short the length of your life limits your potential for good. It prevents your efforts from ever coming to a successful conclusion. The only successful human is the one that advanced the species during or because of his or her life. Failure is the wasting away of potential. Realization of potential is good.
Theft is bad.
It doesn't matter whether the stolen thing is an idea, an object, or the affection of another, it is evil and has the effect of causing strife within the species and will result in any or all of the other bad things. It can be the cause for Murder. It is a form of abuse to another member of your species. It can be the cause of hate for those you perceive as different from yourself. It is ignorant, a lie and a self destructive habit.