It's quite the opposite of being unserious and unambitious. It's actually being more mindful about our choices. They don't understand because it's unintuitive at first glance.
For me, it's being serious about what I have and need, being focus on my destiny and ambitions. Focus on what I need to hit my goals in life and leave aside the meaningless things that doesn't contribute to our goals. As you see in this statement is clary the contrary.
I understand why they say it, but you are the only one who is in control of your life, so take the wheel and drive 😁