I guess there is no official documentation on how levels work
.
Essentially, level increases when you buy stuff (poses, places, clothes, valuables). It may also increase as you add lovers or spouses (and decrease if you remove them - I think this is the only way level can decrease).
Likes used to increase level in the past ( and Dislikes, when they existed, decreased level ) but I'm not sure if it still happens now.
Some claim that rooming also increases level
but I'm not sure either.
Maybe Support could clarify all of us