Life has been pretty good to Bert and John Jacobs lately. The Boston brothers designed their first t-shirt in 1989 and spent the next five years hawking their wares in college dorms and at street fairs, surviving on peanut butter and jelly sandwiches.

As the brothers said, "Chicks were not impressed."

Fast forward to today: their brand has exploded since they created their iconic brand character Jake in 1994. Life is Good products are sold in over 4,500 retail stores across the United States and are available in 30 other countries. The brand built by media darlings the Jacobs brothers is now worth $100 million.

How did they do it?

Here are 5 things that drove these inspirational brothers to insane startup success:

1. They were unbelievably tenacious.

"After 5½ years of selling T-shirts, we had $78 in the bank," Bert Jacobs told Fortune magazine in 2014. Why did they keep going? They were making enough not to have to get jobs, he explained, and it was delivering on their dream of combining business and art. In short, they were doing what they loved. They still are, and after decades of working side by side, that's pretty amazing.

2. Their core values are evident in everyday operations.

The Jacobs brothers remain committed to simplicity, humor and humility. They say, "Through Life is good Festivals, positive products, and a steady dose of ping pong, Jake's crew does its best to keep the good vibes flowing." In fact, Bert is known as the company's chief executive optimist, while John is the chief creative optimist.

3. They believe in something bigger than themselves.

Their desire to counter the onslaught of negativity in mainstream news was a huge driver for the Jacobs brothers. The brothers told Forbes, "On the road, we had a recurring conversation about how we wanted to counter the daily flood of negative news -- this led to the creation of our first Life is Good shirt, and it really took off."

In 2012, they told ABC News, "...optimism, we've learned, is at its most powerful during trying times. People need something to help them keep their chin up. And you know, buying a t-shirt or hat isn't going to break a household."

They didn't want to change the world, but had identified one issue they could do something about. It's an incredibly effective strategy for startups.

4. They learned from their failures.

It would have been simple for the brothers to throw in the towel after an unsuccessful stint on the road in 1994. They could have given up the dream, applied for "real" jobs and moved on. But entrepreneurs often learn our greatest lessons from our failures -- I know some of my biggest insights early on came from rejection letters from investors. The Jacobs brothers learned from their unsuccessful road trip and came back better than ever.

5. They shun conventional wisdom and go their own way.

Incredibly, the Jacobs brothers have eschewed advertising and instead rely on the good vibes and social power of their community to spread the word about their products. At the May 2015 ICONIC conference presented by Inc. and CNBC, Bert Jacobs told attendees, "We don't have to scream about Life is Good. We don't have any signs there that say Life is Good [at our events]. What's the halo effect to our brand? I think it's better than interrupting them to talk about ourselves, which is what advertising has been for decades and decades." Often, it's that unconventional strategy and counterintuitive thinking that sets the insanely successful businesses apart from the moderately successful ones.

T-shirts existed long before the Life is Good brothers came along, yet they've been able to succeed in this hypercompetitive market, even building their business through recessions. While we hear a lot about tech startups and the revolutionary things they're doing, innovative startups don't always have to reinvent the wheel or change the world.

Sometimes, the most successful startups are just finding a new way to solve an old problem--or in the case of the Jacobs brothers, a new way to make people smile and spread good vibes.