One is born a dog because one had “dog gati” and had done actions suitable for “a dog”. For example, dogs don’t pay attention to civility or “good behavior”. They pee and poop anywhere they like. They have sex with their family members, etc.
On the other hand, one with “deva gati” or “human gati” will be disciplined. They enjoy giving, helping others, doing good deeds.
Birth (jati) is within a given existence (bhava), and bhava is due to one’s gati. This is very important to understand.
It can be seen at various levels.
Those who are born in “good realms” (deva and brahma) had “good gati”. Those born in “bad realms” (peta, asura, animal, hell) had “bad gati”. Human realm is a “good realm”, but humans can CULTIVATE both good and bad gati. Then their next bhava is according to their cultivated gati.
There is a lot to think about what is stated above. See:
“Gathi and Bhava – Many Varieties”
“Gati to Bhava to Jati – Ours to Control“
