Suppliers supply. The criteria for entering such a relationship is fairly simple. Do they meet our quality standards? Can they supply at a price acceptable to us? Are they reliable?
If these criteria come back positive, a supplier relationship is viable.
Partnerships require a different set of questions. Are our brands and values compatible? Does a business relationship further the purpose of both organisations? Are we comfortable to have mutual brand associations? Can we unlock synergy value? How will we address conflict?
Go on a few dates first before you get married. The glossy marketing brochure is not a fair representation of reality. With suppliers, you end contracts. With partners, you incur brand damage.