Lily and Marshall have it all figured out - they happened to meet each other at a young age and never strayed. They’re happily married, but they appear to age quicker than the unmarried Ted. (Example: the episode where the couples at the bar are sitting and complaining while the singles are on their feet and on the go.)
Ted is constantly searching for the love of his life, but in that, he has more adventures than it ever seems Lily and Marshall have. He has the gooey, lovey-dovey couples experience, but he also has the ability to go out with Barney, get wasted, and have one-night stands. Although he so desperately wants that once-in-a-lifetime love, his journey on the way is the whole premise of the show - clearly the story worth telling.
So even though the characters seem to ache for what Lily and Marshall have… is it the true ideal?
