As with all other aspects of fiction, the key to writing good dialogue is honesty.
Only enemies speak the truth; friends and lovers lie endlessly, caught in the web of duty.
If God gives you something you can do, why in God's name wouldn't you do it?
Because things like this you can only ssay once. And you either get it wrong or right, it's the end either way, because it's too hard to ever try to say again.
The trick is to teach yourself to read in small sips as well as long swallows.
If you want to be a writer, you must do two things above all others: read a lot and write a lot. There's no way around these two things that I'm aware of, no shortcut.