Sometimes when you meet someone you purposefully hide your flaws just to try to impress the other person... After awhile when that friendship feeling turns into a love feeling and those attributes stop being so hidden it could throw the other person out of love...
Another option is that the two people fell in love too quickly without really knowing the other, and when things were known that previously hadn't come up yet, dissuaded the person to not be in love anymore...
Or maybe the two people fell in love without living with each other, or knowing how they live, and once it came time to move in with each other the undesirable traits outweighed the desire to tolerate them for the sake of love...
I'm a firm believer that if you truly love someone you're willing to make it work... If you fall out of love with someone because of how they live or what I've described above, you may be too emotionally immature to be in that serious of a relationship right now (I only suggested these as options because it's usually what ends up happening (Occam's Razor and whatnot))... First learn to communicate, then learn to cooperate/compromise, and lastly learn to love...
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