Reward players early on and let rewards increase as the game progress. Receiving gifts or being rewarded for gameplay brings happiness and this, in turn, makes a player return to the game again.
We created a visionary case study for Angry Birds 2, including game collectibles along with the game storyline.
First rewards would unlock already after beating the 1st piggy boss. The more levels are finished, the better the rewards get. This stays true to other games as well.
It’s important to scatter encouragements along with the game storyline. Harder levels deserve extra motivation, like “Only 2% get’s this far, here’s an amazing reward waiting for you!”. It’s great because in Angry Birds 2 case, the game is played by tens of millions of players daily and being in the very coveted 2% club gives the extra boost to continue the game.