Far from grilling you during the interview, the hiring manager barely asks a single question. In fact, he spends more time trying to convince you that you need to take this job. Then pop -- you've got a job offer on the table.
What's wrong with this picture? That's what you need to ask yourself. An overly eager hiring manager can only mean one thing: desperation. Maybe he's interviewed dozens of candidates and they've all turned him down, or the job is commission-only and he's got nothing to lose. Alternately, the company could be a start-up with a very small bank account that needs to hire as many people as possible who are willing to work for pennies.
If the interview seems too quick, ask a few questions of your own. Find out how many people have already been interviewed for the position. Ask what the job pays, and how it's structured (for example, salary, salary-plus-commission, or commission-only). Also find out what happened to the last guy who was in the job. If he quit after a couple of weeks, that's a big red flag.