I'm not going to comment on the Republican love of deficits, but from a progressive point of view, it's fairly obvious why Democrats might behave this way.
Liberals don't love govenment spending for its own sake - we support it when and because it actually does things that we think are worth doing. So, if it can stimulate the economy, educate children, put out fires, build infrastructure, keep people alive then we want the government to do those things. But we also realise that there is a limited amount of money that can be raised from taxation. If it's necessary to go into debt in the short term to keep the country moving, then that's the right thing to do. But in the long run, every dollar that we spend on interest for loans is a dollar we're not spending on health, education, poverty reduction or innovation. So it's in our interest as progressives to find a stable way of funding our programs that is sustainable. That's why we want to reduce medicare costs. That's why we want to cut waste from the Pentagon. That's why we consistently support Pay as You Go policies.
The 5% of the federal that we spent last year on debt interest was not wasted money, but it wasn't maximally efficient either. In the long run, we want to stabilise this in a way that doesn't cause people too much pain in the short term.
But Republicans... just don't care about this at all. Not even a little bit. So ask them why not...