Interests are what defines the Sim's likes and dislikes in MySims and are for the player to know what a Sim likes in their furniture and buildings, as well as what to avoid putting. They will have a loved interest, a liked interest, along with a hated interest.
The loved interest in a Sim is the interest that the Sim loves and is interested in the most. When giving a gift that has their loved interest, you will be given more relationship points than just giving items that contain their secondary/liked interests. Sims' personalities are also more defined by their loved interest. For example, a Sim with a love of Spooky will have horror music playing near their house, despite also having Cute as a secondary Interest.
The secondary interest is an interest that a Sim will like, but less than their loved interests. The player will still receive relationship points when giving items to a Sim with their secondary interest, but not as much as giving an item with their loved interest.
The hated interest is the interest that the Sim despises. You will lose relationship points with the Sim if you give an item that has alot of their hated interest. However, if their loved interest percentage in the item dominates the hate, the Sim will not deduct any relationship points, but they like the item a bit less.