You know things are bad when you see an American flag flying at half-staff, as I did last week, and you’re not sure why — not because there seems no reason for it to have been lowered, but because there are so many reasons.

Belgium? Pakistan? The day-to-day onslaught of vile political rhetoric?

(I know we don’t lower flags for that last one, but it’s a thought.)

So I was relieved that on that same day, I’d be getting an emotional boost from Shane Lopez.

Lopez is an author, psychologist and senior scientist with Gallup, but his finest talent is swimming upstream.

Despite international terrorism, homegrown gun violence, political brawls and poverty, Lopez isn’t losing hope in humanity, and he doesn’t want us to lose hope, either.

“Hope is contagious,” said Lopez, author of the 2013 bestseller “Making Hope Happen” and a soon-to-be-released book about “dream jobs.” (Hint: They’re made, not found.)

All we need to do to catch the bug (and we want to catch this one) is find ourselves what Lopez calls a “hope mentor.” These special people are old and young, rich and not so rich, conservative and liberal, introverted and extroverted, but they share one common trait — “a tremendous amount of energy for the future. They’re a group of super-empowered individuals,” Lopez said. “They’re born with a leaning toward hope and they pack it on as they grow.”

Sometimes hope mentors are famous. Lopez singles out writer Dave Eggers, for example, for his inspired nonprofit ScholarMatch; the program mentors first-generation and low-income high school students throughout the college application process.

But more often, these folks are living next door. “Out and about every single day, you’re probably bumping up against a hope mentor,” Lopez said. “We know that half of people are hopeful, and half are not hopeful. Every other person we meet is one of those hopeful people.”

They’re the library director who gets a new library built. “Libraries are hubs of hope,” Lopez said.

They’re the city worker “who makes neighborhoods beautiful again.” They’re grandparents and teachers and doctors.

They’re sometimes still in high school.

The most hopeful group is, in fact, people in their midteens to late 20s, Lopez said. “That makes sense. They’re at the beginning of their adult lives.”

But even those of us who haven’t been in our 20s for quite some time can practice hope — for our kids and grandkids.

It’s important to do this because — cynics, listen up — hope is more than just a nice idea. Research has found that having hope leads us to eat and sleep better, exercise more, have safer sex. Hopeful people have fewer colds and are at less risk for diabetes. Students with hope for the future get better grades.

Lopez has been taking his hope-mentor message on the road because a growing number of people want to hear it.

“There’s a greater sense of uncertainty in the air,” Lopez said by phone from his home in Lawrence, Kan.

“People are more concerned about their future, their children’s future, the nation’s future. The politics are so contentious, so derisive, that people are really concerned about which way is up.

“They’re not sure they can trust what they are hearing, not sure any candidate can move us ahead in the right direction. We’re desperate for someone with the guiding touch.”

So, let’s say we’re ready to be guided. Do we just walk up to someone and ask, “Will you be my mentor?”

Actually, yes, Lopez said.

Hope mentors, by nature, want to share their good feelings about the future. And they have little trouble ignoring the naysayers and doing rather audacious things to better society.

“Watching them will teach you how to behave in a positive way, how to push again, despite setbacks,” Lopez said.

“You might say, ‘You’re the most hopeful man or woman I know and I need to spend some time with you.’ ”

Then, like a flag, watch your own hope meter rise.