Wizet felt he was fortunate. By the time he began studying "sympathetic magic," he already had a relatively deep understanding of the soul.
Not only that, but he also had his magic eyes, which gave him a unique perspective from which to observe wondrous places such as the Shadow World.
The unusual experience of studying the Shadow World also helped him better understand "sympathetic magic."
Even without relying on his research into souls or his magic eyes to understand "sympathetic magic," he could study it through other types of magic.
For example, the Animagus transformation, an advanced form of Transfiguration, could be used to study the principle of "similarity" in "sympathetic magic."
The animal an Animagus transformed into was generally considered a physical manifestation of the soul.
Peter Pettigrew, whose soul had already been sucked out, was one such example. He had once sacrificed a finger to cast powerful dark magic, blowing up a street to cover his escape.
That finger had been lost to dark magic, and Peter Pettigrew had lacked the means to remedy the damage and recreate it. As a result, the loss of that finger had been reflected directly in his soul.
When he maintained his rat Animagus form, one of the rat's front paws was also missing a toe. This was a manifestation of "similarity" in "sympathetic magic."
If someone mastered both the Animagus transformation and the Patronus Charm, the corporeal Patronus they summoned would usually take the same form as their Animagus.
This happened because a corporeal Patronus was also a physical manifestation of the soul, and could likewise be considered an expression of "similarity" in "sympathetic magic."
Voldemort, meanwhile, wanted to truly hurt Harry, which was why he coveted Harry's blood and wanted to incorporate it into his own body.
In Voldemort's