Suppose that my value is 100 and I bid 110. Let b be the highest offer by a rival.

If b 100 < 110:

Then overbidding does not matter: I win and I pay b.

If 100 < b < 110:

I win and I pay b > 100. Overbidding reduces my payoff (I would get 0 by bidding 100).

If b >110:

Overbidding does not matter: I lose whether I bid 110 or 100.

Exercise: use a similar argument to show that I never want to bid less than 100.