Friendship Level and its Importance
Building up relationships with animals is crucial as it dictates the amount and type of raw materials you gain from them every time you complete their requests. The heart icon with a number is the indicator of Friendship Level (with that particular animal), and the gauge increases every time you perform a request, as well as simple stuff like giving them gifts or talking to them.
Every time your friendship level with an animal friend increases, you will also receive some sort of achievement rewards in the form of Bells, Crafting materials, and during the latter levels some essences. At the same time, each filled heart will be added to your character's experience gauge (Player Level).
Additionally, you get to unlock or earn special rewards from an animal friend when you manage to reach a certain level of friendship with them. These rewards come in the form of personal items like clothes or Furniture. When you reach the maximum friendship level with an animal friend (usually level 20), they will give you a picture of them.
Improving Your Friendship Level with the Campers
To improve or level up your friendship level with an animal, the basic idea is to simply interact with it like talking or taking on their requests. Each animal has also some kind of affinity to certain Amenities which when built, improves your friendship level with the affected animals. Although the best way to crank up your friendship level with an animal is still to perform their requests. This is also very productive as you get additional rewards after performing the request -- which will then be used as Crafting materials to make better Furniture or amenities. However, there is a limit to the number of requests each of the campers can ask per day. Should you want to perform more of their request (than the regular limit), you'll have to use a special item called "request ticket" to push through.
Crafting Amenities / Furniture
Crafted Amenities and Furniture on your Campsite aren't simply ornaments. The actual idea is to build them in order to unlock level limits of the animals that has a special affinity to them. Not limited to this, when you welcome an addition to your amenities, every animal attending the grand opening will receive additional boosts in their Friendship Levels (apart from the boost of the amenity to particular animals which share the same affinity). Although it may sound highly beneficial to you, you should also know that Crafting amenities and furniture costs a lot of Bells, Crafting Materials, and sometimes even Leaf Tickets -- which is why it is rather a much difficult method to use in increasing your friendship levels with the campers this way.