It is important to keep your players' moods high, as bad mood will negatively affect your players' training. A player looses approximately 2% training per mood level under level 7.

To keep a player's mood up, the player needs to earn mood points. Players earn 1 mood point for every minute they are on the pitch in matches.

A player loses mood points when he misses out on on-pitch time during their squad's league matches. Mood loss is based on the player's age, his current mood level and which squad he is in. Mood loss happens slower the lower his mood level is. The younger the player is, the less time on the pitch the players will demand. On the mood page you will find a table that indicates how many mood points the player will loose each minute. The mood loss will be less the less the mood level is. Below you will see an example table of how a player's mood is affected. The example shows a 16 year old player.

Mood Graph

As shown in the table the player will loose 0.5 mood point for each minute the player is not on the field during a match. If the player sits out he will loose 45 mood points.
In the graph above you can see how many points a player has gained or lost by hovering over the circles in the graph. You will also be able to see how much it will take to get back to full mood points.
When you buy a new player he will always have 740 mood points which is the maximum amount.