In fluent speech, when a /s/ sound is followed by a /j/ then the /s/ changes to a /ʃ/ sound, and this change also happens if the next word begins with the /ʃ/ sound itself. So 'bless you' becomes 'bleshyou' and 'this shirt' becomes 'thishirt'. This is an example of assimilation. And don't forget this can also happen if the /s/ is followed by a /t/, because as you might remember from a previous video, the /t/ sound disappears between two consonants. Here are some more examples.

1. Bless you.

2. Can you just shut the door please?!

3. This yacht is beautiful.

4. Don't worry, that's usual.

5. He always makes you feel good.

详见：https://www.bbc.co.uk/learningenglish/english/features/pronunciation/tims-pronunciation-workshop-ep-21