Simplify the following text for a Grade 8 native English-speaking person.
Original: According to Da Vinci 's contemporary , Giorgio Vasari , '' ... after he had lingered over it four years , left it unfinished ... '' It is known that such behavior is common in most paintings of Leonardo who , later in his life , regretted `` never having completed a single work '' .
Simplified: According to Da Vinci 's contemporary , Giorgio Vasari , '' ... after he had lingered over it four years , left it unfinished ... '' .