Human nature tends to be selfish. At some point, our individual needs and interests will be revisited and annoyances will not be tolerated as easily as in the first stage of love. Life also gets hectic with other responsibilities and commitments and it becomes harder to get that good quality one-on-one time.
In the first stage, you may have wanted to spend all your time with each other, but as you enter stage two, it’s normal and healthy to want time separate from your partner.