Asking why we should avoid contradictions sounds like asking why we should avoid venomous
snakes, or asking why we should avoid stealing. But contradictions aren't dangerous, and they aren't
immoral. If the question is taken to mean 'why is it dangerous to contradict oneself', then the answer
can only be 'it isn't always'. And if the question is taken to mean 'why is it morally wrong to contradict
oneself', then the answer is 'it isn't ever'.
But when we say that we should avoid contradictions, what is usually meant is 'in rational argument,
we should avoid contradictions'. And this is true, because what we mean by 'rational argument' is
(something like) 'being logically consistent', and what we mean by contradiction is (something like)
'being logically inconsistent'. And so the answer is, IF we want to engage in rational argument, then
we should avoid contradictions.