You may use 이것이 넌 내 친구인 이유다. (Or you may use more formal style) Also u may say with 그래서: 그래서 넌 내 친구야 Variant with 때문에 is also possible, as you've said. :)

this is why you are my friend = 이게 니가 내 친구인 거야 actaully, that whole sentence is a abbreviation so origial basic fromal sentece is 이것이 니가 나의 친구인 이유이다 this = why -> 이것 = 이유 and this is why you are my friend = this is the reason that you are my friend right? so your sentence is also abbreviation too if you wanna learn about abbreviation, then you should learn about basic formal form first if you do opposite, then... you gonna be confused pretty often ^^