As Bambi has stated you have to interact with it. The other way of keeping it happy is buying buying Magic Soda Pops to keep up your sprite's mood and Magic Honey-Glazed Biscuits to keep its stamina up. You can find these items from any Sprite Merchant approximately in Kaslow(X 220, Y346), Jale(X 242, Y187) Or Ilya (X 355, Y 73). They will be located on those three cities under this symbol
. Jales Sprite merchant name is Evet, Kaslow's is Hamu and Ilya's is Yobellia.
Another option is the Sprite Helper System. To get to this system press "K" on your keyboard, Click on the "SAY" tab if not already there, Then go to the second page and look for this symbol
, Right Click it and then go to the "Sprite Supply" tab and set it up accordingly. This system will only work for your guadian sprite(the sprite you chose at the beginning of the game), other sprites(the summoned ones) you will have to maintain yourself.