It's the scariest thing ever to realize how much someone means to you.  When it hits you, all these thoughts and questions run through your mind at once.  A sad emotion even starts to creep on you slowly, inch by inch, as you start to wonder.  What if things don't work out?  How are you possibly going to live without them?  Someone that was once a stranger is now the person you know like the back of your hand.  Someone you once had no emotions for, now has the power to break your heart.  Someone you never used to hang out with, now owns most of your time.  Someone you never thought you'd love, now owns your entire heart.  Someone you once lived without is someone you now wish to hold onto forever.