Probably not Arrogance...

Maybe work on strengthing your boundaries...then you don't have to worry about trying to change his behavior...just change yours

Example: If he continues to ask why, attempting to convince you that this needs to be more of a priority...simply let him know that you will think about it tonight and get back to him in the morning. In the morning, just let him know that you considered it, but you are not able to...basically make your no stick...if he continues to ask just repeat what you said...you considered it, but you are not able to. If he doesn't get that you are have made your decision...then you probably need to let him know that you don't think he is hearing you...

The details...ask for them up front...simply tell him that you would like to get the details before making a committment for the volunteer opportunity...let him know you have a concern as sometimes it feels like you don't know what I'm getting into...