I don't work in the airlines, but I'm a private pilot with lots of friends and family who do work or have worked in the airlines (and in the aviation industry in general).
Expenses are high and often unpredictable, and profits are low. A lot of airlines operate on incredibly thin margins, where only one problem could push them into bankruptcy. Back in 2008, Frontier Airlines had to go into bankruptcy (and was ultimately acquired by Republic) due to a dispute with its credit card processor. http://en.wikipedia.org/wiki/Frontier_Airlines#Bankruptcy_an...
Competition is also fierce and is almost entirely price-based. While there have been a few attempts at competing on other metrics (there were several attempts to start business-class only airlines in the late 90s and early 2000s) have almost universally failed. In fact, the only one I can think of currently is OpenSkies, and it is backed by a major airline (British Airways).
It's also (as you would expect) a very complex regulatory and legal environment. Simply getting off the ground can take years of work.
I have idly thought about a small airline that would connect a handful of smaller towns across the Southeast that lack affordable or convenient air service to larger airports, where they could interconnect with the majors. But when I even begin to look at the numbers, despite what an exciting idea I think it could be, I know there is no way it would be successful.