Human penises, much to the relief of most humans, lack the "barb-like structures found in many mammals" known as penile spines. For many of us, knowing that we will never encounter a barbed penis is enough; for scientists, who are the guy at the party saying, "I mean, sure, it's a nice present, but have you checked out its teeth?" the human penis's curious lack of spines is a question to be asked, and answered, and told to everyone.
Which is to say that now, thanks to a team of geneticists, we can answer with some degree of confidence the question: "Why don't I have a spiny penis?" The reason, as with all things, is genes; essentially, we humans lack a set genes intended to provide some kind of regulatory coordination to a "male hormone-signaling gene"—genes that are present in chimpanzees who, yes, have penile spines.
A similar stretch of regulatory DNA located near a "neural development gene" in chimpanzees but missing in humans would seem to have given rise to large brains, which we have since used for things like the cave drawings at Lascaux, "Las Meninas," and NCIS, none of which chimps, likely to their credit, have ever bothered with.
"Hmm," you are likely thinking. "Tell me more about these penile spines." Take it away, BBC:
Penile spines are barb-like structures found in many mammals. Their role remains under debate, and they may play different roles in different species.
They may increase stimulation for the male during mating. They might also play a part in inducing female ovulation in a small number of species, but there is evidence that they can cause damage to the female too.
Then there is the suggestion that they might have evolved to remove "mating plugs" - material that some male species deposit in the female genital tract to block other males' attempts to fertilise the same female.
Any more questions?