It is said when we affirm a fact or action and make it clear that we will not be part of it. And, after time, we see ourselves in the need to be part of such fact or action. Examples : I would never fall in love with a person like that. . . and after a while that person is our boyfriend (a) and sometimes husband ( a) . Similar to this expression is "a talker falls sooner than a lame one."
it is something like O.E. : never have to say never, because at the least thoughtful moment "that" can happen to us. Another maxim to explain this sentence would be "By mouth dies the fish". Because if you're talking, you're going to have to swallow your words at a certain time. Another saying that might serve to explain the case a little more is that of "Never spit up". . . because the spit could fall back into your mouth.
