'One's own personal gain' means no more than it says. We all strive to benefit or profit from our activities - GAIN from them. 'One's own personal gain' is simply that gain belonging to oneself. Below is what happens only when selfishness or inverted intentions toward others is employed to attain or increase 'one's own personal gain'.
When we become very selfish to the extent that for our own benefits we stop bothering about the losses that other will incur because of one's own selfish intentions.