In most cases, I have left one job to explore new challenges and avenues of work rather than any dissatisfaction with the job. Make this clear to your employer. Keep those bridges intact in case you ever need to travel that way again. Taking care of these previous relationships will help you to take bigger risks since you have someplace safe to retreat to should the new position not work out. You will feel less stress in the new position and worry less about personal issues.
Startup companies can be exciting and frightening places to work.
It is up to you to take advantage of everything a startup company has to offer while avoiding the pitfalls that can and do occur. After the move Last week I talked about not burning bridges to your old job in case you need to return there. This week I will concentrate on the other end of that equation, negotiating with the new company for the best possible deal. Make sure, before your start, that you understand exactly what salary you will be paid, health care insurance arrangements, working hours and environment and try to understand the mind set of the principals in the company. This effectively cut my salary by the same amount.