Phinjo and Chong Cha,

Thanks for writing so much. It's good practice and well done. Please look at the changes I have made to your sentences (below) and compare them to the original.

I made changes in the sentences to clarify meaning. 

1) Italian scientists have found that the chemical in the brain which makes  us romantic disappears over a year and that the start of a relationship does not last forever. Researchers from the University of Pavia discovered that levels of a chemical called Nerve Growth Factor(NGF) increase when people fall in love. NGF levels come from the rush of adrenalin and love of life that occur when new love blossoms.   
  
Research Leader Dr.Enzo Emanuele reported: "At first, levels of NGF are elevated among people in love. They found increased levels of NGF in the new romantics and that NGF caused sweaty palms and butterflies in stomachs.

(2)Nerve Growth Factor(NGF)   
 (3)Lasts only a year                                                                                                                                   (4)There were 58 volunteers. Yes, there were in love.
                                                                                                                                                                (5)The research was conducted by Italian scientists. They found differences between the volunteers. NGF levels come from the rush of adrenalin and love of life that occur when new love blossoms and the chemical fades over a year or so after people become more secure in a relationship.
                                                          
 (6)increased levels of NGF. when people in love.