How would you, as someone involved in the hiring process (manager,interviewer, etc) feel about a candidate that has changed jobs every 1-2 years?

updateThanks for all the input everybody, some really great responses, and good info in every post.  I asked it because I'm currently at my 3 job in the last 5 years and I'm feeling like my position is going nowhere (like the position should have been contract in the first place, not full-time).  

My only options here seem like transition to a different team doing something I'm not really interested in or look for new work, but I'm a little afraid my recent job history is all short stints.

It depends on the context:


In a startup culture (like Silicon Valley), one to two years is the lifetime of many companies, and it's expected you'd be switching your place of employment that often.
If you're a contract worker, a contract may only be a short, set timespan.
Everywhere else, one to two years is an unusually short stay at a company.


In any context, employers are generally looking for a person who's going to be in it for the long haul, whatever the long haul is for the company:


Startups are looking for someone who will last until the exit: acquisition, IPO, shuttering, etc.
Contract hires should be able to successfully complete their contracts to term.
Other companies are looking for an employee who will last long enough to make a return on the investment of hiring them: this can take several years.


It's a red-flag to potential employers if you're constantly leaving your job for personal reasons, even if you have perfectly valid reasons.

I'd also note that having experience in one context isn't necessarily going to translate to another. 

For example, if you're a life-long contract worker, it can look just as unappealing to a company looking to hire full-time employees as someone who went from regular job to regular job. Similarly, a person who stayed at a job for 10 years might be unappealing to a startup that wants people who are constantly looking for the next big thing.
