The best thing to do to increase friendship and to most likely get gifts back is to send gifts!
Here are a list of things you can send:
These are all unlimited gifts Valentines Gifts
These gifts are items you own! So be careful what you give and who to!
All the animals in the game are available to give as a gift but some of them are not unlimited gifts and some are one's that you own!
To view the all the animals click here