Here's why:
* Phonetic rules: French pronunciation is heavily influenced by the sound of the following word. This is called "liaison" (linking).
* "S" is silent: When "s" is followed by a vowel, it becomes silent and the vowel sound from the next word connects to the previous one.
Example:
* "Je aime" becomes "j'aime" because the "s" disappears and the "a" sound from "aime" connects directly to the "e" sound of "je."
Let's compare this to a word with a consonant sound following the "s":
* "Je parle" - the "s" is pronounced because it's followed by the consonant sound of "p."
So, you are spot-on with your observation!