We're not actually spending a "deficit". What we're doing by burning fossil fuel is reducing a potential energy gradient we've discovered - turning the stored energy into work and waste heat. In other words we're maximizing entropy, and in the process using the "work" to (temporarily) increase the order in our own, human system.
The "purpose" of life, as far as the universe is concerned, is to allow entropy to be maximized faster than it can be by non-living systems. And intelligent life maximizes entropy as fast as humanly possible.
Maximizing entropy as fast as possible given the constraints is what the universe is all about.
You might be interested in reading the background for my current thinking. This is a comprehensive, but fairly straight-forward 1997 paper by Rod Swenson, discussing the 4th Law of Thermodynamics that he discovered in 1989, and how it's responsible for creating order on the universe, from bottom to top: http://rodswenson.com/humaneco.pdf
In terms of my lifelong search for root causes, reading this paper earlier in the week was the greatest "Eureka!" moment I've ever had in my life. Even more than losing my virginity or my first acid trip. But then, I've always been a little strange, don't ya know...
Post script for those who are interested in life's twists and turns. The author of the above paper, Rod Swenson, is a legitimate ehro of science for doing this. He was also a rock and roll impresario in the 1980s, who produced shows for the likes of Blondie, The Ramones and Patti Smith. He came up in the NY punk scene that formed around CBGB's, and was the man who created the Plasmatics and Wendy O. Williams. Talk a bout a polymath...
Here's an interview with him that covers both the evolutionary aspects of his theory and his revolutionary days with Williams and the NY music scene. It also has some cool performance pics of Williams to whet one's boyish appetites and set the hook for the serious science it contains:
http://www.vice.com/read/revolution-evolution-and-rock-n-roll-an-exclusive-interview-with-plasmatics-founder-rod-swenson