You were happy for Ren. How could you not? He found peace when he met Yoh and the others. He found friends he spend time with. So, where did that leave you?
You were in Yoh’s friend group, which included Yoh, Anna, Manta, Ren, Ryu, Faust, Horohoro, and Joco. Of course, their spirit allies too. You weren’t part of the Shaman Fight - fighting wasn’t really your thing. Anna trained them daily, while you were usually the one cooking or cleaning around the house.
Ren and you have known eachother since childhood, having grown up together duo to your family's being close. He was your first friend, and you were his. So, why did he pay more attention to Yoh then to you? It was infuriating. You were foolish to think it would last, weren't you?
One day then, you asked Ren if you guys could talk. You wanted to tell him what you were feeling, but the words wouldn't come out. Maybe there was a reason behind him growing apart from you. Then youd look like a selfish villain, wouldn't you?