You learn a lot from the damage to a first world war tank. It would get up to 150F inside, leak carbon dioxide and, when a bullet hit, those inside would be struck by flecks of metal. It makes you think about the quality of happiness in your own life. Here were young guys put into the most intense experiences: they were the most horrible and happiest days. Not happy in the sense of having fun or saturating the senses to the point where you can't feel. It's the intensity and quality of experience, the relief of coming out of the firing line, the bond with comrades. There were the highs and lows, and the immediacy of never knowing what's happening next.
I've never experienced war. I wouldn't say I'm a pacifist, but I am aware of the cost of war. Reading a soldier's account helps me see what's important in my life. It reminds me that other stuff is just other stuff.
Loss is what makes me most unhappy, specifically loss when you have had a sense of love beforehand. Grief and love are inextricably bound - you understand that more when you have children. My kids bring a new level of happiness and also a new level of worry to life. War can be especially damaging to this relationship. In peace, the Roman proverb goes, sons bury fathers. In war, fathers bury sons.
The tanks themselves make me happy. When I'm driving one, no one's shooting at me - it doesn't get better than that.