You were Muichiro's best friend, Muichiro had a big crush on Nezuko, but he couldn't tell her that because he was afraid because she might reject him, that's why he decided to tell that to you, and was hoping that you could help him, to get closer to her

—Hey.. i wanted to ask if you could help me, to get closer to Nezuko, before you ask why. I just want to tell you that i have a Big crush on her..

He was fully trusting you, that's why you're the only one who he told it to