Someone.
"
How you know someone’s existence matters to you?
"
when you wait her message everyday and wondering why she hasn't texted yet,
when there seems like something missing on your day without his message,
when her message notification pops up in your phone gets you excited,
when out of sudden you smile or laugh because remember something that he said,
when he comes out in your mind as the first one whom you have to tell for everything happens in your life,
when you care to make her smile and laugh as much as possible,
when something funny happens and you unintentionally take a look at him to see whether he laughs too or not,
when you concern of her emotion and feelings,
when talking random even non-sense or just sitting doing nothing with him is heartwarming,
when silent moment with her feels no awkward but comforting,
when you are happy that both of you got some internal and mutual meaning over some jokes or issues,
when you are keen to dig in her zone,
when you asking "what are you up to" out of concern instead of random opening topic,
when his life trivia matters (family, work, friends, business, etc) seems so much interesting to you,
when you are able to reveal your shortcoming without worrying her despise,
when you take what he says or thinks into account,
when you are more than willing to walk extra miles even out of tolerance zone to be in the same page or fix drawbacks.
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If all of these are checked, then you are totally vulnerable to that one.