Jupiter is known as the happy planet, the beneficial planet, the planet of luck and optimism and all of that good stuff. At his core, Jupiter is the planet of expansion. He expands whatever he touches. While he does want to be positive, that doesn't always mean that the expansion that occurs will be positive. He can expand whatever is there, whatever is happening, so he may expand on the good, or he may expand on the bad.
What does transit Jupiter expand?
The house that transit (moving) Jupiter is touring in your natal chart gets a big dose of his expansive energy. You can find new opportunities, open up to new possibilities, and open your mind and world. But you can also see that when things go wrong, they get worse in a hurry, and if things aren't going well when Jupiter enters the house and you're not doing anything to right the ship, he can make it much worse.
When transit Jupiter is aspecting a natal planet or house cusp at a critical or anaretic degree, this can expand the critical energy associated with that planet, and the difficulties you may normally contend with regarding that planet can become more pronounced. It can be in your face. But since Jupiter is the happy, lucky planet, if you work on doing things right and making things better, he'll help you do that.
When transit Jupiter is making a hard aspect - a square or opposition in his case - this can expand on the difficult energy of Jupiter. There can be an excess of everything, particularly laziness and indulgence, and you can go to extremes and not really feel it. It doesn't seem so bad to you, because Jupiter makes you feel so good even when things aren't going well sometimes. But again, because Jupiter is the happy planet, you can work with the squares and oppositions so they're not as difficult as with other transit planets.
Conjunctions by Jupiter are generally not seen as hard aspects, but they can be incredibly expansive. It's usually in a good way, but it can be in a difficult way too.
In your natal chart, the house and sign location of your natal Jupiter gets this expansive energy throughout your life, and it can expand on the good or expand on the difficulties you experience with the areas of life ruled by the house and sign. You can turn things around more quickly, but you may slip into quicksand more easily as well.
When transit Jupiter is retrograde, expansion may be more limited, and we can all feel more limited with the opportunities we have for expansion, or lack motivation and drive to find them. When transit Jupiter is at a critical degree, we can also struggle with expansion, and may go too far or not far enough.
So remember expansion when assessing the impact of Jupiter in your life, and focus on doing things right so that even if things are difficult, he can help you to make it better.
Suggested Reading: Progressed Jupiter in Astrology