Well, accoding to statistics, nearly one fourths of the whole population have the same problem with you in the world, and it i explained that the reason is resulted from genes. Sometimes people call this allergy to sun light. Basically, it does no harm to our health, but it still get a lot of people puzzled. So, you don't have to be worried about that, since you have millions of peers who get this. Anyway, the root cause of this is sitll unknown, but it will be discovered.
Most people doesn't have that kind of issue, but there are about 10 to 35 percent of the population sneezing in the sudden presence of bright light, especially sunlight, even though scientific and popular attention has largely focused on that issue, but it's still hard to explain why. Some people said that sun sneezing is a gift of God, so if your friend sneeze next time, tell him thanks God, haha, just a joke!
The reason of sneeze is just because his obesity figure, or partial obesity, e.g. tonsil. Few people may be caused by the abnormal of nasal parts. The obesity person's air passage of respiratory system will be narrow than average people. If his Pharyngeal organization jammed when he rise his head to looking at the sun in the sky, the air pass through the narrow place of the nose, the nose and pharyngeal parts shake, the sneeze will happen. The abnormal nasal parts will lead to nasal system narrow, the flow choking lead to sneeze. It seems nothing relative to the eyes and the sun, but head rise.
Usually people having this symptom are genetic endowment. The sneeze is because the nose is affected by the foreign matter which should be out of the body. The eyes and nose perception are dominated by the same of trigeminal nerve, stimulating a strong defense responses confusion together. That is why it causes the appearance of sneezing when stimulated by the sunlight in which there exists a lot of invisible bacterium.