There are two ways that you can get your buddy to be excited.

Poffin

The fastest way to get your buddy to be excited is use the new item Poffin. You can purchase this from the shop for 100 Pokecoins. The benefits to using Poffins are

• Completely fills your buddy's hunger meter
• Automatically boosts their mood to excited
• Automatically earn the maximum number of hearts in the feeding category. This includes the additional three hearts you can obtain while they're excited
• Your buddy will stay full and excited longer than when they become excited through activities (see below)

Activities

You can increase you buddy's mood by simply interacting with them based on the activities listed on the buddy's profile page. The following activities are:

• Walking - You can earn up to 3 hearts a day (6 if they're excited). Your buddy must be visible on map to earn hearts

• Feeding - You can earn up to 3 hearts a day (6 if they're excited). Feeding your buddy berries until provide you 1 heart whenever you fill up the hunger meter to the max

• Playing - You can earn up to 1 heart a day (2 if they're excited). For this one, all you need to do is press "play" on the buddy menu and pet them for a couple of seconds until there is the visual that they are happy.

• Battling - You can earn up to 1 heart a day (2 if they're excited). Battling in Gyms, Raids, Team GO Rocket Battles, or Trainer Battles will provide you the hearts

• Screenshots - You can earn up to 1 heart a day (2 if they're excited). Simple take a screenshot of your buddy from the buddy menu to earn a heart.

• New Pokestop/Gym - You can earn up to 1 heart a day (2 if they're excited). Simply just visit a Pokestop or Gym that you have not visited before to earn a heart. (This might be actually bugged right now. I received a heart after visiting a gym I had visited before)

• Bonus - You can earn up to 1 heart a day. You can a bonus heart when your buddy gives you a present (required Great Buddy), gives you a Souvenir (requires Ultra Buddy), or by visiting a Pokestop or Gym they wish to visit (requires Ultra Buddy)

As you do these activities, your buddy's mood will steadily increase. Their mood does not appear to be based on the number of hearts it obtains, but by how often you interact with them. So the more you partake in these activities, the better mood your buddy will be in. The activities and hearts reset at midnight local time.

Do note though! Their mood can decrease overtime, so always interact with your buddy as much as you can.

Once your buddy becomes excited, additional perks become unlocked. These perks available as long as they are excited:

• Earn twice as many hearts in the given day (excluding bonus category)
• Walking distance for candy is reduced

The following is a chart of the different mood levels, where Excited being the last one with hearts around it.

Source