If something is done excessively and give negative impact to your life, that's addiction. That's the less stricter definition of it. And many of us are dealing and/ having a form of addiction.When I say choose your addiction wisely, it means considering the risk of doing it.
Because in order to achieve something there's always something to risk for.

Here's an example : if you choose to get addicted to Studying, you will risk probably social life, health and relationship. But this addiction may open door of opportunities not to mention, financial gain too and knowledge.
Another example is if you choose to get addicted to trading. You will risk money,social life even relationship But, this addiction may bring you more money. More money may mean freedom for some of you and better quality of life.
So, what about addicted to something that is deemed harmful? an addict usually use any justification to justify their behavior. So, I am not going to be telling you if you should stop your addiction. But I just want to emphasize on thinking through it. Is it aligning to your goals in life? is it something that you truly want and will make you happy? will you regret it or will you not?
~ signing out
Addiction of anything never gives you a positive result it always gives you negative result,,,
Your example of studying excessively is nice!!!
Everything in our life must be balance whether it's any habit or any thing...
For balancing our life we must need to know the answer of that WHO AM I..
It's about choosing the sacrifice. Studying excessively -> no social life -> poor health.
Actually addiction of something could be harmful but it has to be related about ruining your life in someway. So in my opinion if studying is your addiction it would not make your life worse...Actually i think it is a bad example for a defining of addiction. Gambling,drinking alcohol smoking are more suitable for example. However addiction has no logic. You just do it without any reason. So we study because we want to be successful in anything that we want.
Out of these in my country,Turkiye,we say "more than everything is harmful."
Have a nice day...
Gambling, smoking, sugar, drinking, these are strict definition of addiction. but I agree more things are harmful. But like I mentioned above, there's less stricter addiction definition that even a good thing may mean addiction.