In your story, if you introduce a main character who is a caring lover, and by the end of the story s/he is still what s/he appeared to be, a caring lover with no secrets, no hidden passions, no dashed dreams, then your reader will be sorely disappointed, or at least bored.
By the same thinking, if your main character’s inner life matches his/her outer life, that is, everything about him/her shows up front, then his/her character becomes repetitious and predictable, hence tedious.
The revelation of a character’s inner character in contrast or contradiction to his/her outer characterization is fundamental to all fine storytelling. With interesting characters, what seems is not what is. People are seldom what they appear to be, and a character’s hidden nature waits behind a façade of traits, good or bad, for the right moment to reveal itself.
Whatever they say and however they compose themselves, interesting characters will reveal their true nature only when placed in a pressure situation. Pressure is key.
Underneath a character’s appearances, are they loving or cruel, strong or weak, generous or selfish, courageous or cowardly? The only way to know is what choices that character makes under pressure. For instance, if a character tells the truth in a situation where telling a lie would gain nothing, it reveals little about his/her inner nature. Yet, if this same character insists on telling the truth when only a lie would save his/her life, then we see his/her true nature.
That’s why a story should have escalating levels of pressure situations going from low to high to extreme. As the pressure builds and builds, your characters should reveal more and more of their inner-selves until the reader knows them body and soul.
Taking this principle a step further, the best writing not only unveils inner nature, but arcs or changes that inner nature, for better or worse, over the course of the tale.