Noun
らくてんしゅぎ
あいつほど楽天主義なやつはいない。
There isn’t a more optimistic guy than him.
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 jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.