I have some questions regarding ULIP?

1)for e.g,Why do people say it is an investment for long-term.Say for e.g,I invest in an ULIP,say Rs 1000/- every year for 20 years, and
at the 20th year,if the market is really down, then it reflects in your NAV and your returns as well right?

2)over a period of 20 years my NAV also varies depending on market trends right?(for e.g NAV for 5 years may be Rs.20,another 5 years may be Rs.100 and another 5 years may be RS.5/-) Is the historical NAV values considered for the returns when the policy is matured ?

Whenever NAV is high, that means good returns.. So, is it necessary to be long term 15-20 yrs?