Numbers are for example. Does it makes sense to engage in such day trading? (For bitcoin)
You only buy an investment if you think it's going up in value
You only sell an investment if you think it's going down in value (or want to cash out and don't want to risk it going down more)
So buy at 30 and sell at 40 is perfectly reasonable. But you'd only buy at 46 if you think it's going up from there.
And obviously it would have been better if you had kept it rather than selling at 40, but you wouldn't have known that it would go to 46 at the time. Don't let past performance (or regret) be the only factor in your buy/sell decision.
Addressing your comments:
I think I did right by selling at 40k and bought more at 46k.
Well, it would have been better to keep it, but there was no way to know that at the time, and I can't fault you for locking in a 33% return.
And I think regardless of how much bitcoin rise, since I now have more btc it will profit me more than it would be otherwise(case where I hold 30k$)
Wrong. Let's say you bought 100 bitcoin at $30 for an investment of $3,000. If you sold at $40 you'd now have $4,000 for a $1,000 profit. If you then bought at $46, you'd only be able to buy $4,000/$46 = 87 bitcoin (rounded) rather than 100. So you have less bitcoin than if you had kept the lot you originally bought at $30. If you had kept it, you'd have a total value of $4,600 rather then $4,000.