Warriors: Fire and Ice (Series 2) by Erin Hunter

Firestar and Greystripe's friendship was so strong,
Firestar grieved and grieved when Graystripe didn't
return from retrieving Leafpool, (who was named after Leafstar in Firestars quest) Firestar didn't want to leave Graystripe when the clans made the great journey, but he had to for the sake of the clans!