Sometimes, the passive voice is very useful. Other times it sounds awkward. I usually use passive if I want the attention to be focused on the object of the action. For example, in the following sentence, the attention is on the dog: "The dog bit the mailman." But if we use the passive voice, the focus shifts to the person, instead of the animal: "The mailman was bit by a dog." Neither option is better than the other. They just direct the reader's attention differently. Also, passive voice can be more poetic sounding, so keep that in mind if you want to write something in that style.

It does sound weird. Sounds very pretentious. As if I'm reading out of a book.

I agree with Sarah.

Of course, it really depends on the context - there are cases where you really should use passive voice, and others where it's better to use active voice. This is a very general question that you've asked, and a yes/no answer won't be helpful. For your examples, "She couldn't keep back her tears" is what you would normally write. "She" is the topic, and I expect the sentences before and after this sentence to tell me what else she did. "Her tears couldn't be kept back" would be difficult to follow up on, and build into a long paragraph. What more could the tears do? However, as a writing device this can be very useful: by describing what happened using passive voice, you've moved the focus from a feeling human being to an impersonal object. In a way, this "distancing" can enhance the emotional level of the situation. I'm not sure if I've lost you there, but basically passive voice (in general) gives the sentence an impersonal feel.