If you were to ask many successful people in this world, they would tell you that surrounding yourself with positive people is essential for success. What healthy means can be different to different people. I am not the position to say who is healthy and who is not. But I do know that having people around you that have the same goals as you is empowering.
People naturally surround themselves with people that they feel comfortable with.
If you want to quit smoking (for example), it will be easier to quit if the people you are with are not smoking. This doesn't mean you should alienate yourself from a friend that smokes but it does mean asking them not to smoke around you if you are trying to quit. If they care about you than they will be willing to respect your wishes or else spend less time with this person.
If you find yourself having a negative attitude about things in your life, really try to be around people that are encouraging and supportive. If you are with people that are attaining the goals that you want, you will be reminded that you too can accomplish those goals!