Your brain is essentially play-doh. You can sculpt it however you wish. If you aren’t on guard, bad habits can easily become implanted, whether through your own doing, or as a result of an external invasion into your mind by others against your will (however in some cases, you allow it to happen unconsciously). If not corrected, over time these habits can magnify and thus become harder to remove. However, good habits can also be implanted, and likewise, if proper care is taken, those can also be magnified and thus harder to remove. No one really knows what depression is.