Day to day the nuts and bolts of the job can be very pleasant, rewarding and enjoyable.
Taken down to the bare bones nearly all commercial flying is basically 'shift work' which is well known to wear people out over time. The deeper problems can lie in the things behind the scenes, large financial burden, short term employment stability issues, a requirement to achieve at near as 100 percent of ability at all times coupled with shift work can make for a stressful old mix for what is basically a flesh and blood machine that is of course human IOS update 1.0 and only capable of so much.
It's a very expensive occupation there is no doubt. The risk burden is being pushed further onto the Pilot workforce; charging for training coupled with temporary/ fractional fixed term employment contracts. This happens in business of course when there is excess capacity and supply (look at air cargo at the minute!). The customer can pick and choose the terms and place greater 'risk' on the provider. In this case because there is an endless supply of keen people with desire to enter the profession. The airlines ever financially driven, successfully and insidiously shift what was previously their own burden further onto the employee or contractor. Unlike a business though, made up of many people, a Pilot has to carry that risk and burden on his todd which can make life quite a bumpy ride sometimes. This doesn't sit well with a job that requires clear thinking. It's been an area of focus on the industry for sometime now.
Just thoughts from myself, of course only my opinion. You'll perhaps hear some disagreement from the odd person sitting pretty in lovely jobs and good on them. But unfortunately a large amount of aircrew are not in that position.