You have to look at the causes of homelessness to understand.
If you're a dietician, visit some of the soup kitchens in your area. Volunteer to help and watch the people and what they eat. Being a dietician will help you understand the nutritional value of the food that's offered and if you talk with the people that go through the line, you'll realize a few things. First is over-eating, because they never know when or where their next meal will come from. Second is that the food available to them often contains "empty Calories". Third is the amount of salt in the food even before they add it to the food. (any idea how much salt you take in on an average day? the average need is 3/4 of a teaspoon a day. [1500 mg] the average person, homeless or not, takes in an average of 3400 mg, mostly from processed foods)
A lot of the faults you see are listed in Maslows hierarchy of needs. Health issues, like obesity and smoking (and the unsaid drugs and alcohol) do not even come into play until the second tier. Even shelter, which is on the first tier, is towards the bottom of that list, behind Air, water, and food. Sleep and clothing are ahead of shelter even. The dog, is an attempt to move to a higher tier, the third, and social belonging. A dog doesn't judge you like people. Lock a dog in the trunk of a car, then let him out after 5 minutes, and he's happy. Lock you wife in the trunk of a car and see how she treats you after 5 minutes.
Tattoos? Many of them are "homemade". If you see a homeless person covered in tattoos, it doesn't mean they spend every penny on them. It means that at one time, they had some prosperity going for them and could afford to have them done professionally. Also, any number of homeless people you see with tatts most likely got them while they were serving in the military.