what contributes to homelessness?

Dad and I were driving on Sunday and we saw a homeless girl and her dog. He pointed it out and said it's a shame. I asked for who and he said both. We talked about it a bit and it got me thinking that maybe some of our ideas about homeless aren't necessarily true. He said they tend to go to the ER rather than county hospital. I figured that may be true as they get a warm meal, shower, and place to sleep.
I am thinking sometimes they end up homeless because of lack of support especially the elderly who can't work or care for themselves because of sickness or disability and the young people starting out who don't have lots of skills. Sometimes it seems like they should be going to employment agencies, looking at ads in the paper. It only costs a few cents to buy a paper from a paper machine or the library as the computers there are free to find work before hitting the streets. Other times it looks like they were thrown out or have mental health/drug problems  both of which make them unwanted.
It seems like there's two different sides and either way it can be hard to find out why the person is homeless unless you know their story