There is a Mitzvah to make the Chosson (groom) and Kallah (bride) happy on the day of their wedding. This Mitzvah is of utmost importance as it is one of the few Mitzvos for which we stop learning torah (another one would be attending funerals).This illustrates the importance of this category of Mitzvos. The question is why on such a stand-alone happy day? After all, the couple is getting married. Is there a requirement for the guests to make them more happy?
To understand why we make the couple happy, we must first point out the way we make them happy, which is by praising them. As the gemara in Kesuvos 17 asks 'how do we dance before the bride? By saying she is attractive and kind'.
The reason to praise the bride and groom is so they should be happy with their decision to get married. As Beis Hillel ends the discussion there by saying 'when someone buys something in the market, should you praise itin his eyes, or degenerate it?'. So the basic idea is that at the time when someone makes the most life altering decision they will ever make, we try to help them be happy with their decision, and not be nervous about it, or have regrets. This is most applicable right away at the wedding and the seven days of feasting. By praising and entertaining them, they ignore their nervous feelings.
In fact, the Radal on Pirkei DeRabi Eliezer chapter 16 #61, points out just how deeply this praising of the couple is connected to praising someone's acquisition. He points out that the gemara in Kesuvos 17a only mentions praising the bride, whereas the Pirkei DiRabi Eliezer mentions praising the groom as well. He explains that the sugya in Ksuvos was discussing whether to praise her with her actual qualities or give a standardized praise was specifically taking into account the fact that the groom was the one who made the kinyan, aquirement, therefore Beis Hillel argued to Beis Shamai that a standardized praise was in order, just in case she is lacking qualities, we don't want him to regret his aquirement.
But, says the Radal, concerning the praise about the groom, Beis Hillel will agree with Beis Shamai to praise him with his actual qualities, because a woman is happy with anything, as we see in Kedushin 41a טב למיתב טן דו.