What is the major cause for this sleeping too much, is it because this person is very lazy?

Probably not. Sleeping too much can be a sign of several problems. One common problem is obstructive sleep apnea (osa), which is when breathing slows down or stops too many times during the night. Sleep quality is disturbed, and as a result, the person with osa may sleep a large number of hours at night and nap in the daytime, but still be sleep deprived. Depression is another cause of sleeping too much.
Not necessarily. Everyone has a different sleep requirement as far as the number of hours needed to feel refreshed. But when the sleep becomes excessive it could mean that the sleep the person is getting is not quality sleep(like sleep apnea - a sleep disorder) or it could be mood disorder (depression can present this way), or even lethargy from an endocrine disorder (like hypothyroidism).