Walls symbolize separation, division, and blocking freedom and movement.
Bridges symbolize connection, interaction, and promoting freedom and movement.
The Berlin Wall is one of the terrible relics of the Cold War. Conversely, the fall of the Berlin Wall is one of the great moments of the late 20th century, quite possibly a harbinger if not a cause of the prosperity of the 1990s.
We should all build bridges, moreso of the figurative type, in our lives. Pay a compliment to someone you wouldn't normally speak to. Spend time in a town or neighborhood or even a country you wouldn't normally go to. Volunteer to help others with whom you wouldn't normally come into contact. These types of actions build a society, by building the character and experiences of the actors involved.
The suggestion of building a wall, especially of the metaphorical variety, is almost always reactionary and motivated by fear. As history demonstrates, the forces of freedom will eventually "tear down this wall," because love will always overcome fear. And it is this core understanding in the power of love which is rooted in Christianity, which I believe Pope Francis recognizes in his statement above.