How much of a drop-in value for your investments can you tolerate? Your tolerance of this risk is a function of when you need the money. This is your time horizon. Usually, the younger you are the less thought you give to fluctuations in the value of your investments. This is the volatility of an investment. You should align your investments with your time horizon.
This is actually what I was thinking about and you perfectly expressed this thought.
While I am young I do not care if I spend, say, $100 for entertainment and bad habbits. Though these $100 transform into $1,200 a year and $12,000 per 10 years - and that's the price of a nice new car, which I might have bought without a bank loan.
Definitely, one should pay attention to such small details since they make up our lives.