The question refers to the “van scene” with Will in Stranger Things season 4. In that scene, Will cries because he is secretly in love with Mike and uses a speech about Eleven to express his own feelings indirectly.
Will’s feelings for Mike
Will gives Mike a heartfelt speech about how important he is to Eleven, but the details match Will’s own feelings, not just hers. Saying these things while pretending they are only about Eleven is emotionally overwhelming for him, which is why he turns to the window and breaks down in tears.
Why it hurts so much
Will believes Mike cannot reciprocate his feelings because Mike is in love with Eleven and focused on saving her. He is also afraid of being rejected or losing Mike as a friend if he is honest, so he chooses to support Mike’s relationship with Eleven instead of voicing his own desires, which adds to his sense of loneliness and pain.