These notes come from sheets. If you don't know, sheets is an SNG expert.

It's all about risk vs. reward. What are you risking if you win compared to what you could lose?

The closer you are to bubbling your calling range decreases which means you can call with more hands. The opposite is true if you are farther away from bubbling the tighter your calling range must be. In other words you can't call with that many hands.

Calling ranges of opponents will change due to their stack size. For example, if your stack can't bust your opponent then he has a wider range to call.

If you can not risk all your chips and they could bust if they call then a push is ok. Most of the time this will be when your opponents have less than 10bbs. You can safely push any two cards from the button and small blind.