Learn English – word for the mental state of laughter

Quarry to use a hunting metaphor, object to be more literal.

Any synonym with the meaning you want will do.

abiding, lasting, enduring, permanent, constant, continual, continuous, persistent, never-ending, perpetual, sustained, uninterrupted, 24-hour, round-the-clock (etc)

Someone who smiles easily and often might be said to have a ready smile:

in a state of completion or preparedness, as for use or action; willing or eager; prompt or rapid.

It is exceedingly common to learn the meaning of words from their use in context, so you are far from alone. The problem is that for many words (like "irascible") the context is rarely detailed enough to really tell you anything about its meaning, so a lot of people misinterpret such words for years before finally looking it up. It's always a good idea to look up a new word for its denotation and its connotation. Add root words to that, and language appreciation really blossoms.