The excesses of life
One of the most important themes in La Dolce Vita is the excesses of life. In the film, Marcello lives a life of excess. He frequently attends lavish parties, spends quite a bit of money, and lives luxuriously despite not having many resources.
Although Marcello seemingly has everything and lives a life of excess and luxury, his life is empty. He can't find true love or happiness or contentment. On the surface, he has everything; internally, though, he has nothing. Beyond that, Marcello's emptiness is reflective of Italy's culture as a whole and the corruption which is plaguing the country.
True love
True love is another important theme of La Dolce Vita. In the film, Marcello searches seemingly constantly for true love. During each of the seven episodes of the film, Marcello begins romances with several women (and continues his relationship with his fiancée, Emma). Like many people in the world, all Marcello wants is to find true love. But Marcello has true love doesn't need to look that far: he has true love with Emma, his loving fiancée.
Love vs. Lust
The dichotomy between love and lust is a central theme of La Dolce Vita. In the film, Marcello frequently professes his love for his fiancée, Emma. To Emma, the two are in a committed relationship and will be married soon. But Marcello is never a faithful fiancée. He has affairs with and lusts after, a number of women, none of whom he seemingly loves. But he never realizes the true love and the real relationship he has with Emma, a woman who he takes advantage of.