Because is you.

Because, this is all I want with you.

Life's tough when problems strikes, when quarrels happened.

Life's tougher when you're no longer there when there's problem, when quarrels tend to stop and when the both of us starts to take a step backward. 

Life's never gonna be easy,

it's all depends on how the two person handle.

Either to go through obstacles and grow old together,

or gave up half way during the journey.  

I'm not giving up because I know nothing's gonna be easy.

You should know, you're the one and only that I put so much effort on.

I've never had such strong feeling until I met you.

Defining "I love you" is simple,

proving it is not simple at all.