There are multiple studies that show that it can lower a person's libido (which is good if you're horny to the point of being unable to think about anything but sex) and it helps your body regulate the production of erogenous fluids.
For men it can also actually help lessen the chances of prostate cancer.
So it's healthy in moderation, like a good amount of other things (caffine, water, red wine).
But too much masturbation can have a number of negative effects such as inflmation of the glans (causing a burning sensation when you pee) and a really low sexual drive (even if you're a normally sexual person). Too much can also cause things like erectile dysfunction, which no man wants to have at the worst possible times.
There's also some things that can go hand in hand (no pun intended) with over-masturbation. Though they are not always related, porn addiction can cause erectile dysfunction during actual intercourse due to the brain's association between your hand and sex.
1-2 times a day is generally the max for healthy masturbation. Though some studies suggest that masturbating 4-5 times a week is actually really healthy for you in the long run.