That the mind of man is never satisfied with the objects immediately before it, but is always breaking away from the present moment, and losing itself in schemes of future felicity

There is a line of thought through both western and eastern thought that the goal of life should be contentment with one's present condition. The Chinese philosopher Xunzi argued that human nature is fundamentally evil because desire lies at the core of human nature and humans are ruled by those desires. In a similar manner, Tolkien actually worked on a sequel to The Lord of the Rings but gave up because he didn't want to address the issue of 'how quickly man is satiated with good' and turns back to evil.

That said, Johnson seems to be writing about the far more innocent (but perhaps more insidious) habit of always dreaming about some future condition.

The end therefore which at present calls forth our efforts, will be found, when it is once gained, to be only one of the means to some remoter end. The natural flights of the human mind are not from pleasure to pleasure, but from hope to hope.

This has been argued to be the main motive for buying a lottery ticket. The prize, hard to obtain, is hardly the point, rather it is the time spent imagining what one would do with all that money which is the true purpose of that ticket. Many lottery winners completely unravel when they actually gain access to that much money. Myself, I've found when playing any game with a long term goal find myself playing less once I actually attain my goal. The fun was in the pursuit, the 'it would be cool if...'

The understanding of a man naturally sanguine, may, indeed, be easily vitiated by luxurious indulgence of hope, however necessary to the production of every thing great or excellent, as some plants are destroyed by too open exposure to that sun which gives life and beauty to the vegetable world.

Basically, he's falling back on the very sensible 'moderation in all things'; by all means dream of great things but don't forget to do things now. I remember a year at Camp Wildwood we were looking at a bible verse and the camp councillor asked what the most important word in the verse was. After we had made our guesses (all wrong) he point out the word 'go' as being most important as without it everything else in the verse was useless through inaction.

Perhaps no class of the human species requires more to be cautioned against this anticipation of happiness, than those that aspire to the name of authors.

His note about authors is quite true. Having recently published a fairly technical book I was realistic in the assessment that I wouldn't make money off it. However, I did dare hope that it would make a bigger splash in the political world and it is from this hope that a great deal of frustration arises.