They came to Marah, but they could not drink the waters of Marah because they were bitter...
Imagine that you walk in to a school and you ask the first guy you see, "How is the food in this school?" He tells you it's inedible.
"How are the Rabbis?" he replies that they are all boring.
"What are the guys like?"
"They're all losers."
You ask the same questions to the next guy you bump into, and he says, "The food is excellent, the Rabbis are amazing, and the guys are great."
They are eating the same food and they have the same Rabbis! How do they have such different views?
It is human nature that when someone is embittered, he sees everything negatively. On the other hand when one is content everything seems great. Whatever state one is in, that is how he views things.
"The Jews came to Marah, but they could not drink the waters because they were bitter." In the plain sense the verse refers to the water and explains why the people could not drink it. Our Rabbis explain homiletically that the reference is to the people - they were bitter. Because the people were bitter, they found fault with the water.
Instead of living a life of condemning and complaining, forever finding faults with others, it is much more productive and healthy to live a life of contentment - then everything automatically seems good.