I'll be there for you.

Since the first day I met you
I knew something great would come
But love as strong as this
Where did it come from?
^^^
I like your opening stanza. It was like a love at first sight. You can feel it and just know it was meant to be.
- I would omit the "I" in the second line .... allowing for better flow and it's really not a needed word.

We barely knew each other back then
But we took the time
Now we're in each others heads
like we can read each others minds.
^^^
When two hearts blend together so perfectly they can feel the others thoughts.
You've stated that perfectly in this part.

I love you so dearly
I cherish every moment we have
I care about you so deeply
Sorry, if I've ever made you sad.
^^^
This part is so very sweet and speaks of admiration and total giving of the heart.
-I'd omit the "I's" in the second and third line.
You will see how much better the stanza will flow. The reader already knows that it is "you" speaking so therefor the "I's" are just empty words taking up space in your stanza.

This is for my Best friend.
I just want you to know I care.
And If you ever need me
Please believe me, I'll be there.
^^^
Beautiful ending, sweet dedicated words.
- I just want you to know I care.
^^^
Just want you to know I care.

I really liked this write! Your heart has whispered ink upon a page with this sweet dedication.