If you put two girls together, both with mousy brown hair, both awkward eighth graders, both with startlingly blue eyes, and both avid collectors of, well, anything, it would be a perfect recipe for friendship. 
If you make them both short, one a little dramatic, the other a little quiet, one a risk taker and loud laugher, one a bit shy and safety finder, the possibilities are endless for the trouble they could get into. But the certainty of haven is sure.
If you mix them well for over 15 years, have them as nearly next door neighbors for a few, move one seven hours away, move them both to another country altogether. Maybe have them share a room, and then keep mixing well for the next few years so they are never in the same place at the same time for more than a few days, you could have a mess on your hands. 
Or you could not. 
It would be good if they are equally matched in all things, so there is no competition, no jealousy, but it would be best if they are unmatched in nearly everything, leaning heavily on one another for what the other lacks. 
Long periods of silence may be necessary, both in the presence of one another and when apart. But oh how sweet it is when they are 30 years old and still whispering under patchwork quilts late into the night. 
It would be very good if each girl still called the other the same nickname no matter how old, and if at the age of 80, when to be called Bean and Lowly would seem embarrassing and childish, they are still doing it. 
It is good when one or both imagine their weddings, the line of girls accompanying always changes save for one another, they always remain. But it is better that one or both imagine their life, the circle of trust, dependability, unconditional love, loyalty, and those who know how to hug the right amount of time with the right amount of pressure, there is always that other girl. 
Not everyone has that sort of girl in her life. 
But I do. 
Happy 30th birthday dearest friend