Sometimes the appointment of a Supreme Court justice is political in nature and not just about the nominee's abilities to examine constitutional law. Justices are human and come to the court with engrained views of the world developed over a lifetime of exposure to all of the institutions and contacts with the world that determine our attitudes and ideas toward one's positions.
The Constitution is a wonderful document written more than 200 years ago. The designers could not have conceived of all the changes that have occurred since it was written. Consequently, in determining its meaning, it is sometimes difficult to examine the words and apply them to the situation at hand. How a justice reads meaning into those words in the context of today, is, of necessity, tempered by the justice's view of the world.
If judges could remove from their thinking any significant personal leanings, we would not have judges who are labeled conservative or liberal. The best we can do is to have judges who have open minds and have not shown themselves to have preconceived views.
It is important that justices be appointed who do not dramatically shift the balance of the Supreme Court if the court itself is going to be accepted as the last word on the Constitution. It is important that the Supreme Court retains its integrity as a fair and impartial arbitrator of the Constitution and the law.