I used to be in love with a Sun/Mercury-Pluto square, and when he was upset with you he'd compulsively insult you. I always knew the reason behind his behavior, so I was the only person who never got mad about it (I'd think, "Wow, what a Mercury-Pluto square moment")... Several of my friends didn't get why he'd do it to them, and were also very sensitive, so they'd just get hurt and think him a bad person. There was an innate distrust of people in him also, and a fear of the world being against him. He was insecure, and always assumed negative things about people.

I think this is a good definition:

The square can give you a harshness of temperament and expression, where your direct, blunt style can often create conflict and confrontation between yourself and others. Even though you prefer to reflect yourself as one capable of handling the 'real situation', you tend to lacka sensitivity towards possible reacions to your potentially penetrating insights. You rarely 'wrap' them up in socially acceptable packages for consumption, but offer them raw.

Your approach to life has a tendency to be extreme, a little suspicious of others' motives and intentions, secretive about your own, and basically pessimistic in tone. Your behaviour can often be unpredictable and erratic, making it difficult for others to take you for granted, and encouraging them to keep a certain distance away from you. You are a little uneasy with close contact in relationships, taking considerable time to develop a feeling of trust and relative relaxation with partner. There is a tendency to try to manipulate and mould any partner so that he or she conforms to your point of view; and often your views become intolerant and fixed, allowing little space for flexibility or change to occur naturally.

Sometimes, in an effort to achieve your own will, you rely upon your strong will-power to dominate others. Sometimes this can succeed, but you are equally liable to apply it incorrectly through a lack of proportion and through insensitivity, creating more damage to your aims and others than you intended. Similarly, whilst your choice of expressive style is straight and direct, you often devote much time towards scheming and plotting, which, again, often works to your disadvantage.

In many ways, you project your own shadow qualities onto others, finding it hard to trust them, yet laying plots yourself; it is unlikely that you have any special ability to motivate others or to become a spokesperson for any group, as you are more of a loner. Towards authority you are ambivalent, making use of any power and authority that you may possess at home or work, yet being quite anti-authoritarian if you hold a low opinion of those responsible positions, particularly disliking a role under any domination by others.

Mentally, you require more self-discipline and understanding to make more positive use of your abilities of penetrative insight; in particular, the way in which you present yourself to others may need to be moderated. More control regarding your thoughts and words can help to avoid any controversy and unnecessary conflict with others, and help a redirection of your powers of insight inward, so that you can observe how you are projecting those negative shadow qualities out onto others in the world when these qualities should be being internally transformed and the energies released for positive applications. Potentially, you have the sort of mind to absorb a variety of positive views and arrive at a new synthesis of the essential values hidden within each of them, and then to express a unifying point or common ground from which dialogue can arise to resolve the apparent divisions. Such a talent could be extremely effective and important if the necessary inner changes are undergone and a redirection of your life results; a temporary phase which you may experience is the adoption of whichever view / belief system is currently in the forefront of your attention, but that at least can reveal flexibility and potential attitude change in you. Dissolving fixed barriers is possible; and the potential for change in your lifestyle and relationships sit he main challenge with which Pluto is facing you.