Pastor Rick Warren of Saddleback Church believes gratitude is the key to happiness. The more people are grateful for everything they have in their lives, then the happier they ultimately are.

Various research conducted by psychologists and sociologists have proven his belief to be true, Warren says.

"If you want to have healthy relationships, then start with an attitude of gratitude. You will be far happier and enjoy your relationships more if you will develop the habit of being grateful for the people in your life," he writes on his website.

Warren says this simple truth is actually the foundation of good relationships. "When you think of the people in your life, is your first feeling gratitude? For too many of us, it can be, 'What do they need to do for me? What problems have we got? What do we need to get done?'"

But the apostle Paul teaches Christians to think differently. When Paul thinks of his friends, he feels a sense of gratitude. He recommends others do the same if they want their relationships to last.

"Here's the challenge: The longer you know someone, the more likely you are to take that person for granted. Isn't that true? The longer you know someone, the easier it is to focus on that person's faults and the bad times instead of the happy times," says Warren.

This is why "The Purpose Driven Life" author says it takes extra effort for people to choose to have an attitude of gratitude for the people in their lives instead of focusing on all their friends and families' negative attributes.

"When we develop the habit of thanking God for people in our lives, it will cultivate happiness and have a lasting and eternal impact on those relationships," he guarantees.