Optimism
noun
You know that 主義 is "-ism." If you add that to being comfortable like you're in heaven then you have lots of optimism. You're sure you'll be okay in any situation!
This is a noun in Japanese but, depending on the context, it's often translated into English as an adjective.
らくてんしゅぎ
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
兄は人付き合いがありませんが、楽天主義だから自分ではあまり気にしていません。
My older brother doesn't have a social life, but he doesn't mind much because of his optimism.
あいつほど楽天主義だなあと思うやつはいない。
I don't think anyone is more optimistic than he is.
フグは評判通りの楽天主義者だね。
Fugu is as optimistic as people say he is.