No job or career is a continuous orgasm. There are aspects of one's work that simply are not that enjoyable, or perhaps just suck. When the scales are tipped and the enjoyable aspects of one's work are overshadowed by the disproportionate number of tasks that are unenjoyable, the purpose of the work gets lost and the meaning fades.
Soon the job is nothing more than a transaction for which you are paid and serves no meaning.
While meaning and purpose are closely intertwined, meaning is always personal. One's purpose at work is usually dictated by others, but must be owned by the employee. The meaning that we attach to the work we do runs parallel with the opportunity that the work invites or allows us to insert our authentic selves. Meaningful work is usually taken personally.
To find meaning in meaningless work we must divorce ourselves from the perception of the work being meaningless and re-assign the work to our own value system. That's where the perception of the importance of what you do, and the purpose of your job will intersect.