How do i teach/help my dog to not be afraid of other dogs?

i have a shih tzu female named molly. Shes 10 years old. My neighbor has another shih tzu who is male and 13 years old. My neighbors went away on vacation and since i know what its like owning a shih tzu we took her in for the time there away. Molly doesnt act fearfull at home. she isnt afraid of most things but whenever theres another dog around or in the house she runs behind me and/or tries to climb up my leg. i pick her up and she clings for life! She is very well trained and so is the other dog. i cant seem to understand why she is afraid of other dogs? she was never mis-treated. and was raised by a nice family before we had her. she also lived with 2 dogs before us and one was a great dane! Does anyone have any ideas of how i can get them to get along while snickers is staying here? Any tips would be appreciated!