Who wants to help me with my exam

What word or phrase is modified by the prepositional phrase in the sentence below? marisol's puppy likes napping under the couch because it is warm and cozy

Dogs make wonderful pets because they us stay happy and healthy. the topic sentence is "dogs make wonderful pets." , is this correct? i keep doubting myself.

Ineed simplifying this thesis and making it easier to understand for the reader (more concise) "no matter how religious one may be, one’s faith tends to change under unexpected and challenging circumstances."