Making friends with the other people on the island is a nice thing to do. You might end up unlocking events, receiving items, and even getting married.
To see how the other people feel about you, go into your Summary page and tap on the icon that looks like a man at the top of the touch screen. You will then reach your Residents page. Each person (a main or sub character) has 10 heart markers that will tell you how they feel about you. Characters who you can't marry will have green hearts, and characters you can marry will have red hearts.
The small heart markers on the Residents page are generally not equivalent to the colored hearts that appear on eligible marriage candidate portraits. Even if the person you are trying to marry has 10 hearts on the Residents page, it doesn't mean that they automatically are at a red heart color.
Just like in past Harvest Moon games, your relationship with the other villager are based on a point-scale. The hearts that appear next to the portraits on the Residents page will give you a rough estimate of how many points you have with that person.
There isn't an accessory in Island of Happiness that lets you see exactly how many friendship points you have with the residents. The Love Bangle does not exist in this game.
The type of gift you give will either increase or decrease your friendship points. Only the marriageable candidates have a Special and HORROR gift. The faceless residents are on a much smaller scale, and you can only increase their FP by 3 measly points with gifts.
Don't you dare litter!
You will decrease affection if you throw anything onto the ground when you are not on your own farm or inside of the mine! Throwing an item onto the ground when other people around will lose 100 friendship points for each item you toss away. Nobody likes a litterbug!
It is easy to neglect your relationships with all 99 people who can live on the island. If you do not talk to a person for 30 days they can move off of the island, whether they belong to the Main Villager group or the insignificant Sub-Villagers. In order to entice the people to return you'll have to complete tasks that caused them to move to the island in the first place.
A few of the residents will never leave the island. You don't have to worry about any shopkeepers, Witch Princess, Goddess, or Shea and Wada moving out and abandoning you.