It seems so simple, so obvious, when posed that way, right?
In the private sector, where individuals pursue their own objectives; hard work, competition, investment, productivity and innovation are engines of prosperity. When it makes sense - when they're confident - wise folks will spend (always within their means). When it doesn't, they won't. Not-wise folks, on the other hand, will spend beyond their means and suffer hard knocks. If they're lucky, no one will bail them out - lest they be robbed of the lesson,and remain not-wise folks.
Now, alas, in the public sector, where individual politicians pursue their own objectives, rents (in myriad forms) paid to special interests are the engines of campaign funding and votes (in bulk). When it makes sense, politicians will spend. When it doesn't, well, in the pursuit of their own objectives, it never doesn't. They will forever spend beyond
As for the corporate sector (which I'm compelled here to distinguish from the private sector), executives pursue their own objectives in two ways: One would be the intuitive - the competing for yours and my business. The other would be the helping the politician pursue his objectives, and, in the process, through some paid-for legislative act, gaining an edge over their competitors. And the more they can crony up to the politician the more egregious the risks they can take, for they'll have become too big (too important) to the politician to fail.
I.e., I suspect it ("too-big-to-fail") is less about risk to the system and more about the personal aspirations of politicians (*all), captured treasury secretaries (Paulson, Geithner) and Fed chairmen (Greenspan, Bernanke).
*Make no mistake, both sides of the aisle are equally culpable!