It's not that easy. Market situation changes, personal/family/health situation changes, political situation changes.
It's easy to talk, but then the real life knocks to the door.
So the rule number one:

"Never invest money that you can't afford to lose"

It's always easy to look back at the chart and go with "what if" but then, there are always two sides of transaction, one that's buying, and one that's selling