Coworker asked for my salary, I told him, when I asked about his he said he didn't want to share

I joined the company 3 months ago, known the guy for the same amount of time. We've been working together ever since. He's been in the company for 5 years, he's my senior. Basically what the title say

I joined the company 3 months ago, known the guy for the same amount of time. We've been working together ever since. He's been in the company for 5 years, he's my senior. Basically what the title says. I feel betrayed for sharing. Hate this industry...