In-house recruiters or hiring managers, except perhaps the most inexperienced or unprofessional, will not press a candidate to explain a decision to decline an offer. If the company really wanted you and is disappointed about your decision, you might be asked if there was anything that could have been handled better during the recruitment process or that would have swayed you, but it is highly unlikely that you'll be asked to provide a detailed rationale for your decision. In the odd case that you are pressured for one, you are under no obligation to bow to that pressure.

As for relationship-building and helping others:

1. You cannot build a relationship with every person you meet. This is not a realistic goal. 2. You cannot help every single person you meet. This is not a realistic goal. 3. People are good at discerning whether an offer to help is empty or legitimate. If you were seeking employment a week ago and haven't even started your new job, you have no business suggesting to the people who interviewed you at a rejected prospective employer that you are now in a position to assist them professionally. Offering such help concurrent with a notification that you have accepted employment elsewhere is, again, an act of extreme hubris and is likely to be seen as such.