This proverb expresses three different emotions: happiness, prejudice and resignation. Those sentiments reach one’s heart in three different moments. Initially we cannot see the negative aspects of a thing that appears to us perfect, but, as time goes by, the defects and the imperfections come up clearly, until what seemed to be perfect becomes something common that disappoints us.
In particular, this proverb is best suited to the people: even though someone may seem to be pleasant and nice, physically or sentimentally, it will always have defects we have to face to and to accept. For this reason, it may result easier to not idealize a person, even though we love him.
To sum up, each pleasant aspect is represented by the rose and so it is our first impression of them, while the negative consequences are represented by the thorns; which are impossible to see until we hold the flower with our hands.