Multiclassing is generally frowned upon by developers, mostly because in earlier iterations of the game there were many things gained just by taking one or two levels of a class. Example here is the 3.5 swashbuckler whom I never saw be taken for more than one or two levels simply because the class granted Weapon Finesse for free at first level.
Most multiclassing options you will see involve preparing a character for a prestige class, or level dipping to gain some mechanical benefit.
Pathfinder really tried staying away from this mindset as it lead to overpowered and unbalanced characters in the face of players who wanted to stay single classed, and as a result most classes spread out their class benefits over time and levels.
Most casters don't multiclass anymore for 2 reasons:
1st is their caster level will be divided, meaning they will likely not be as powerful as they could be but rather be half as powerful as they should be twice. You sometimes see a one or two level dip, but beyond that there are not many full multiclasters.
2nd is the fact that concentration is no longer a single skill that the character can max out, and is tied to caster level. In 3.5 and earlier a caster who maxed his concentration would be able to make the same concentration for his 5th level wizard spells that he would make for his 1st level cleric spells. Now that Concentration is tied to CL, it makes that option even less attractive than before.
That said of multiclassing, there are still some viable options out there, most common ones include level dips into either sorcerer or oracle to gain the benefits of their bloodline/mystery.
I see a lot of Nature Mystery Oracle dips on Paladins so they have the option of stat dumping their DEX so they can allocate more points into their CHA, and there are several other examples.