We don't live in the past where you picked a company and built your relationships and career around it for most of your lifetime. That's what my grandfather did, but it's not how the economy works today. And I see this as a good thing; not only is the human race more productive by having more liquidity in the job market, but as an individual it frees you up – albeit sometimes more forcefully than you like – to shift around until you find the best place for you.
I enjoyed 4 out of the first 5 companies I worked for and thought I had things pretty great. But if I had stopped there, I wouldn't have arrived at the company I work at now (number 6) nor had the wisdom to know that I should commit to staying here for 3x longer than any previous company.