What is wrong with him, guys?

in february a new room mate moved in with me. we have had a great relationship with her and her boyfriend he is always around sleeps with us. me n him are tight he tells me things she dont know. i have always thought of him as a brother until two days ago when he came around and my room mate was not around, he came and slept on my bed ( i am used to that ) but what he triede to do then infuriated me. how could he. now he ruined our friendship why do guys do this