I work in Company A and I'm kind of the star there (I feel it is mainly due to political reasons). Recently, I received an offer from Company B and I decided to accept it. HR department in my current company pushed me to review my decision and propose conditions under which I would like to stay at company. 

I feel that offer from Company B is much better for my career (project is more interesting, company is also a better suit for me), but I don't want to burn bridges. How can I respond?
['https://workplace.stackexchange.com/questions/78863', 'https://workplace.stackexchange.com', 'https://workplace.stackexchange.com/users/-1/']
First of all, you don't have to tell Company A anything except the last date you'll be working for them.

However, you *could* say something to the effect of

> 
> I see this as a personal growth opportunity and a chance to experience different corporate cultures and different ways of doing things. I've thoroughly enjoyed my time at Company A (*don't say this if it isn't true!*), and may consider working here again some time.
> 
> 
> 

The idea is to spin this as "some years from now I'll be more valuable to you if I have this experience than if I stay here" - kind of like taking a really long sabbatical. Just don't be specific
> 
> I decided to accept it.
> 
> 
> 

If you've already made your decision and accepted the other offer, it's OK to say that.

> 
> I have already accepted the offer from Company B and thus cannot consider any counter offer. My last day will be XX/XX/XXXX.
> 
> 
> 

The expectation is that once a job offer is accepted (in writing), that that's it. You wouldn't accept an offer and keep interviewing or keep negotiating the terms so it shouldn't offend your current employer that you aren't going back on your word to Company B. It isn't burning a bridge to be honest here.