Each planet in your chart has a job to do but some planets are better placed than others to do it (albeit not always without some risk).
Consider that Aries is the zodiac sign associated with confidence, courage, bravery, assertion, and competition. Since the job or planetary purpose of Mars is to compete and win, it’s not difficult to see how, in Aries, Mars is well-placed to achieve his aims.
But in Libra, where Mars is considered to be in detriment (i.e. struggling), he might not fare so well. In Aries, Mars was free to disregard the needs and feelings of others when in pursuit of his goals. But in Libra, the needs and feelings of others are shoved in his face.
This is because Libra is associated with achieving balance and harmony, especially in romantic relationships. As most marriage counselors would agreed, typical Libra traits like diplomacy, tact, and a willingness to compromise are useful to further relationships. Yet compromise (much less tact and diplomacy) does not come easily to Mars. Remember how Mars is hard-wired to win by any and all means? How on earth could that ever play out well in Libra?
Equally, Venus in Aries (the zodiac sign where she is in detriment), will struggle. Imagine how difficult it might be to achieve harmonious relationships through the guise of egotistically fueled assertion and constant competition?
For an example how this might play out, consider the Greek myth about Jason (typical Aries) and the Argonauts and the disaster wrought in his marital relationship with Medea.
By contrast, Venus is very happy in Libra, the sign opposite Aries and one of the two zodiac signs where she is considered in dignity.
The planetary purpose of Venus is to both attract and be attracted ‘the other’, especially when it comes to romantic relationships. In this regard, a willing to compromise is clearly a bonus. Yet all too easily compromise can turn into paralyzing indecision.
For an example how this may play out, consider the Greek myth, The Judgement of Paris.
Venus also does well in Taurus, the other zodiac sign in which she is in dignity. Like Libra, Taurus is characterized by romantic relationships albeit of more earthy and sensuous nature. Taurus adds to the mix her famous loyalty and dependability; such traits usually considered to promote harmonious relationships. Yet when it comes to the pursuit of pleasure, another important of job for Venus, things can get quickly out of control in Taurus.
For an example of how this might play out, consider the Greek myth about the wife of King Minos and her dalliance with the Minotaur.
The sign opposite Taurus is Scorpio, where Venus is in her detriment. Although like Taurus, Scorpio is also loyal, Scorpio is also notorious for being distrustful and suspicious, traits not usually associated with happy relationships. Like Aries, Scorpio is also ruled by the planet, Mars. This means that the relationships of Venus in Scorpio are in many respects like those of Venus in Aries – fraught with competition. Add to that the propensity to be jealous, hold grudges, and seek revenge, and it’s not difficult to see how Venus in Scorpio may not enjoy same quality of relationship as might Venus in Taurus .
Not all is lost, however.
As noted earlier, another key purposes of Venus is to full her desire for sensuous pleasures. In Scorpio, sexuality is not taken lightly are passion and intensity are the order of the day. The problem comes when in the pursuit of pleasure societal taboos are ignore and broken. This is always a risk with Venus in Scorpio.
For an example of this how this might play out, consider Greek myth about Medusa.