What can you consider to be a meaningful life?
There's what you do for yourself and what you do for the rest of the world.
What you do for others:
First, you should at least leave things as good as they were before you came. That's probably a good minimum to shoot for, but it's a bit like "don't be a criminal" in the sense that, yeah of course you shouldn't ruin stuff.
Second, if you can improve something, even if nobody knows it, the aggregate benefits of everyone doing that will be pretty cool. This scales down to your everyday life, picking up some crap in your house as you pass by instead of leaving it, making your house cleaner and nicer without any real extra effort. You might never be a hero, and that's fine.
What you do for yourself:
You need to have goals, and work toward them.
You can do more if you improve yourself; the strong, the experienced, the knowledgeable, and powerful are able to impact their lives and everyone else's more than the weak newbie can. This means being inquisitive. Curious. Develop a taste for adventure.
Strike a middle ground. If you really think extremism is necessary, ponder carefully when to shift back to moderation and make that shift effectively.
Examine your life. I don't mean to include a whole lot of Platonic weight to that statement; just think about things. Think about government and law, about society, about technology, about religion and spirituality and religious organization. Think about history and the future. Think about right now.
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TL;DR: Basically, don't just float through life. Life can be meaningful even if you don't work hard and examine it. But it's like a sprinter who might be able to run quickly even if he doesn't train hard and doesn't watch himself run. You don't know if you're just deluding yourself and you don't know how much better you can be.
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Question:
What is the worst thing you would do to someone who killed the one you loved most?