It's ok to pass with 12 points and a balanced hand
When we first learn to play bridge we are taught that 12 points is enough to open. But you do not have to open the bidding and sometimes it is better to pass.
Should we open with this hand?
We've got 12 points here so we can open, but it's a pretty awful 12 points. What I don't like about it is that we've got ♦A, ♦K, ♦Q but nothing else in the diamond suit. So although it's three tricks, those honours aren't helping us to establish any extra tricks in the diamond suit. We really want our points in our long suit, not our short suits.
Let's make a small change, adding a diamond and taking away a spade.
It's likely that the ♦5 is going to be a trick now. If partner has a few diamonds then we can play the top diamonds and the opponents may well run out of the suit. Points in our long suit make the little cards easier to win tricks with.
We're also vulnerable, so I don't want to end up competing or encouraging partner to compete. It's just not really a hand that I'm that enthusiastic about so I'm just going to pass.
Communicating with partner
Now you might be thinking, well, hang on. I'm supposed to communicate to my partner and I'm giving the wrong message if I don't open. Well, an opening bid shows some number of points, but it also communicates to your partner that you actually want to be playing the hand somewhere, and I don't. So I'm downgrading this hand, it's 12, but I'm going to call it less than 12. Unless partner can do something, then I'd rather be out of this hand.
You don't have to open if you don't want to. You can upgrade some hands. You can down downgrade some hands too. Sometimes it's better to be cautious.