That's peach...

A child that can stand and walk is no longer breastfeeding (or at least shouldn't be) this particular case might be different, but I was taught this in school and I was talking in general...

It's called the oral phase. Children explore the world with their mouth, especially when teething they start biting. The mannequin in this pic has tits so it is only natural to try for a child without shame (which develops at around 4 and up

Skin to skin contact also plays a role, I actually went to school and got taught this stuff as an adult. There are a lot of psychological and physiological factors that come into play to combine into this

Breastfeeding releases a certain hormone in the brain of the baby, which in turn does give a child a very strong bond with the mother stemming from the association of the mother with this incredible hormone and feeling calm and loved. It's basically natural conditioning as opposed to pavlovs artificial conditioning