When you love someone, unconditionally and without restraint, you want to spend all your time with them. Being with that person alleviates a need, an ache that is experienced by your whole body. When they leave, the ache returns, and nothing else brings relief. When this intense longing is felt in every part of your body, it is difficult to appreciate the need for boundaries, but they are essential to the success of your relationship.
I'm sure that your beloved enjoys your company and delights in your affection. But, they can't appreciate your attention if they have constant access to it. You have to be absent to be missed, yearned for. If you are always available, your time together is not rare - and that makes it less valuable. It's like diamonds: if they were easy to come by, and cheap to purchase, they wouldn't be precious.
On top of that, everyone needs time to devote to themselves and the hobbies or interests they don't share with you. Whether it's going snowboarding with friends or sleeping in late, the time is an investment in their happiness and wellness. Without that time to be themselves and do the things they love (that you don't love so much), they become lost and bitter - just as you would. If they feel that you are to blame for their being lost, they will come to resent you - and that is the beginning of the end.
If you want your love to last, you need create space. This time apart can feel unbearable, and it's ever harder to tolerate if you are sitting at home just staring at the clock. So, go out and do your own thing. Fill your time by grabbing a drink or catching a flick with friends, or go burn off some energy with some exercise (I find going for a walk or run is the MOST effective way to eat time and beat stress - especially with some great music cranked on the mp3 player!). Heck, sit on your hands if you have to - do whatever it takes to create some space between you and your beloved - your relationship can not survive, or thrive, without it.