Wow, there are many reasons that make you want another pair of glasses. For example, if you think your glasses are too old, you can change them.
If you think you need different types of glasses to match your clothes or different occasions, you can fill yourself more glasses. But maybe the most important is that the prescription is too strong or too low for you as your eye vision changes. So, try to tell whether it is not fit for you anymore.
There are some tips which will help you. If the glasses are too strong, when you watch with this pair of glasses, it is hazy. And you are not look at the right place or thing. The second is it makes your head vertigo when you wear it. If the prescription is too low, you can't see the things clearly. So, you can go to the eye doctor to have a test with your eye vision. Personally, I think people wearing glasses should go to test the eye vision at least half a year once.