Because there is something wrong with the nose pads or that your glasses need to be readjusted. You needn't quit it. If your glasses keep rubbing your nose, you can use a silicone nose pads which are softer and not easy to slip. The oils accumulation on the pad can be abrasive after a long time, so clean the pad and see if the glasses still rub your nose. Adjusting the ear pieces or temples by slightly bending the ends of them inward is also applicable. This action will help to pull the glasses higher up on your nose. If the measures above don't help, you might as well turn to your optician.
The problem bothers many people who wear glasses. Especially in the summer, when people sweat, the glasses cannot help falling down. If the bridges of their noses are short, there are rarely any ideas to help it. However, there are some tips to adjust your glasses. Firstly, you can tighten the spectacle frames to make it hold your glasses more steady. Secondly, see if your glasses have nose pad. Your glasses may rub your nose if the nose pad of the glasses are not completely touch the nose. You can change a new one. Usually, the dull polished nose pad is better because it can increase the friction between your nose and the nose pad. Finally, there is a thing called nose pad sticker. It can be pasted on the surface of nose pad to increase the friction. If your glasses still keep falling, I think you can consider changing glasses into contact lenses. I hope these can help you.
You will feel bad if your glasses just keep rubbing your nose. You should adjust the position of the nose pad to make it higher or lower. You don't have to quit the glasses. You could just have a try on this and see the effect. If it is not worked, you could adjust the position of the eyeglasses frames to make the distance between your side of the lenses and your side of the face limit to some way. You could just use the screw to do the change.
I think you must have choose a pair of heavy glasses. Your frame must be heavy or your lenses are of high power and low index. The weight of your glasses would all be put on your nose and makes it much easier for your glasses to rub your nose. You won't feel your glasses are rubbing your nose if it's very light. Besides, maybe you've gotten a pair of glasses with incorrect PD. The PD of your glasses may be too big for you as to make you feel your glasses are loose and would rub your nose frequently.
To solve this problem, you should check the weight and PD. If it's too heavy, you should quit it. If it's too loose, you should change a tight frame.