Sharing is hard to get just right-- share to little, and you're considered stingy. Share too much, and it can just be a danger to yourself.
Oversharing can give the impression to people that you're reckless with your items, or that you'll just give it away. People could also take advantage of you-- if you're too generous, or too nice, they might use that to take lots of favors/money and never return it, or to use it against you. Oversharing, especially to the wrong people, could also gain you enemies, as people might think you're trying to manipulate them by being "too nice". We were raised thinking that everything has a price, and oversharing could make people think that you're trying to harm them.
If you overshare, you could also give away too much of your own things, or own work. If you're too nice and "overshare" your test answers, it could lead you to being expelled for cheating.
In conclusion, oversharing could be much more dangerous than being stingy, and you must always be careful with what you have, and how people might use you to their advantage.
Hope this helps!