Usually due to lifes pressures building up that creates a non-defensive mechanism that makes you indifferent.
Not to be confused with someone who's been a loser their whole life or the holidays; it IS a random phase.
Dick: "Yeah, I stopped going to the gym and I just eat and drink whatever I want. I just don't have it in me. I guess I'm goin' through a fuck-it phase."
Bob: "A 'fuck-it phase?'"
Dick: "Yeah, I just don't give a fuck. Don't worry, I'll snap out of it at some point in the future...hey, can you pass the ranch!"