You don't need a reason, spoken or unspoken, to say no.
When you were born, you reserved the Divine-given right of refusal--not for life and its experiences--don't mistake me, but for anything that compromised your integrity, your truth, your beauty, your expression, or your values.
Challenge them? Sure, why not? You're strong enough and you know you love a good challenge. But just to fight for fight's sake? No. You don't need that expenditure.
You are here to challenge the system--the one put in place around you. When you lead from instinct, from compassionate heart, you will always know when the answer is an echoing, resounding, undeniable yes.