Why does Graystripe switch clans?

In the original series, sometimes known as Warriors: New Life, Graystripe falls in love with a RiverClan cat Silverstream, and they have two kits. But due to no medical help, Silverstream dies shortly after delivering her kits. Graystripe agrees they belong in RiverClan where their mother was born and raised, but he is torn and decides to leave and go to RiverClan in honor of Silverstream. He also chooses to leave ThunderClan because he wants to raise his own kits, and not have to later fight them in battle. 'Fireheart, Fireheart, I'm being torn in two!' was what Graystripe told his friend, the main character, after making a hard deciscion of leaving his birth Clan to join his kits. He later returns to ThunderClan when he and Fireheart were forced to fight. He wants to be with his kits and Thunderclan doesn't trust him after Silverstream and him