Unexpected behavior when assigning values to dictionary entries

Please, can somebody explain why is the behavior in the example provided and what is the best way to avoid it:

If I try this it works as expected:

var1 = 'original'
var2 = var1

var2 = 'changed'

print(var1)

Out ---> "original"

BUT if I try the same on a dictionary:

var1 = {'data': 'original'}
var2 = var1

var2['data'] = 'changed'

print(var1)

Out ---> {'data': 'changed'}

Why does changing the value of var2 modifies the value of var1 when using dictionaries, but not otherwise?

Thanks