Se la pasa

[aleCcowaN]

As Rusty says, in these cases the pronouns carry meaning themselves and doesn't relate with the object, hence the term lexical/lexicalized (intended to enrich the lexicon).

You may imagine lo to be the time in pasarlo bien (have fun / enjoy oneself/ves) or la to be the whole situation in pasarla bien (same meaning).

Many times these pronouns are part of longer verbal phrases and they don't even coordinate gender or number, like in Argentine expression ponerle el pecho a las balas (literally, to use one's chest as a shield, meaning to be fearless in adversity or carelessly imprudent when in danger). It should be ponerles el pecho ... because it's las balas, but it's not. So you have to learn these as vocabulary.

Fortunately it may be just 15 or 20 you should know to reach an advanced level in Spanish.


ETA:


I don't know where to find a list of cases. They are a subset of verbal phrases, and there are glossaries for them. Regional language provide tons of examples, for instance, Mexican "a ver cómo le hace para solucionar el problema"



Some instances that come to mind:


pasarla bien/ pasarlo bien
vérselas ---> vérselas con alguien (to deal with someone) / vérselas negras (to face a great deal of troubles)
matarlas callando ---> to be up to something on the quiet / to appear innocent