You've got your dream job. All that work during the search process has paid off: The constant refining of your resume, the interview practice, the obsessive research to find the perfect position at the perfect company.
You got the gig...now you just have to keep it.
Situations change over time. You might feel comfortable now, but decisions outside your control could impact your position. Companies sell out to competitors. Management regimes change. Customer tastes evolve, and some firms are too slow to adapt.