People aren't static entities. Everything that happens to you, every choice you make, it defines who you are and changes the person you become. There are infinite timelines with billions of copies of you in them, but you're the only one that has your unique experiences. It doesn't matter if they're good or bad, or if you remember them or not, they still shaped you into the person you are today.

That's why it matters.

[There's certainty in his voice, like this is something he's thought long and hard about in the past. Being so well-versed in timetravel helps to put some things in perspective. Even the parts of his life that he despises and regrets, if he removed them then he'd certainly become a different person as a result. He used to think of that as a good thing, but...]

If no one experienced hardships... perhaps they'd become selfish or ungrateful. They wouldn't be able to appreciate the value of what they have.