What us the reason for having Sheva Bracot week after wedding?
Thank you for your question.
The reason we have Sheva Brachot is in order to rejoice and make festive meals together with the chatan and kallah to make them happy. It is a mitzva to make the chatan and kallah happy, and therefore we even make a bracha when rejoicing with them. This is the basic reason for Sheva Brachot. The amount of Shave Brachot depends on the level of the happiness of the couple, therefore if it is a second marriage, when the happiness is not as great as a first marriage, then the Sheva Brachot period is shorter.
Another reason given by the Chofetz Chaim is that when the blessings of Sheva Brachot are said, because we are praising Hashem while saying the Sheva Brachot, and it brings the divine presence and bring blessing upon the couple. This says the Chofetz Chaim is why it is important that the Sheva brachos be done in a way that conforms to the laws of tzniut, because we want the divine presence to stay there.
Ran Tractate Kesubos 7.