It was all through you that we lost game. ==> "It was (all) your fault we lost the game." or "We lost (the game) because of you." He failed at his exam through not studing good enough. ==> "He failed his exam since he didn't study enough." or "He failed his exam because he didn't study good enough." I've dreamt of becoming a flight attendant since my childhood. ==> "I've always dreamt of being a flight attendant" (This is better because saying 'always' means you've been wanting to be a flight attendant since you were young.) Why do you want to bocome a flight attendant? I want to become a flight attendant since it'll give me a chance to travel around the world. ==> "Why do you wanna [want to] be a flight attendant? I wanna [want to] become a flight attendant because it'll let me travel around the world." (It's better to use 'because' here) My greatest dream is traveling around the world. ==> This is correct but to make it more natural you could say biggest instead of greatest. If you have any questions just ask!

It was all (because of) you that we lost (the) game. He failed at his exam (by) not studying enough. I've dreamt of becoming a flight attendant since my childhood. (correct) Why do you want to become a flight attendant? I want to become a flight attendant since it'll give me a chance to travel around the world. (correct) My greatest dream is traveling around the world. (correct)