2. If the bitcoin has a good future ahead.
3. Buying bitcoins if there is something to predict on the market numbers.
I always want minimum risks so I look good into the market prices of the last half year and other numbers that count for the bitcoin, if these numbers are good I mostly buy them.
If you look at the numbers right now plus you keep in mind that the halving is coming you should buy your bitcoins for sure, right now.
That is very general principle. It is difficult to put into practice. We need the precise point to buy/sell.
It is really difficult to judge when to buy and when to sell, as we cannot be sure that what would be the perfect price either to buy or to sell, totally depends on luck.