I came to the realization that software engineering was not what I wanted to do - about 4 years ago. 6 years into my career now. I still do it because I have bills to pay. I sometimes wonder how many others are in my shoes.
I would say a combination of lifestyle and mental aspects. I do enjoy programming still, just not everything else that comes with it. Agile, meetings, business requirement documents, more meetings...
Something I have had trouble with is finding purpose - I work at a government agency, and I feel my work means next to nothing (for various reasons I can't get into).
I did manual labor for a while during university, and the sedentary nature of my current job may be contributing to my decreased interest to carry on with it.