Pride - When is it appropriate?

Today in line at the bakery we were all admiring the beautiful pastries. The woman in front of me stated “As much as I want one, I’m going to pass it up today”. The woman behind me said “I’m so proud of you. Good for you!”. These women are strangers to each other. It struck me as very odd because I wonder how can you feel pride for someone you don’t even know. I feel pride in myself, and my husband and children. But I know myself/them well and have a vested interest in their success. I also know what takes effort for me/them. I feel pride in my country and in the members of our military, but that feels like a birth-right. When do you think it’s appropriate to say “I’m proud of you” to someone you don’t know?