Sources in daily contact with the late "Friends" star tell TMZ ... Matthew had "never been happier" and had just moved into his new house 3 weeks ago and was loving the new digs.

In addition to being happy about his new home, we're told Matthew had a lot going on in Hollywood ... our sources say he had recently committed to acting in a new movie, a drama, and he was also working on other projects and coming up with ideas for scripts.

Overall, our sources say Matthew's outlook on life was optimistic ... and he was sober and felt content.

Add it all up, and it makes sense why those around Matthew are so shocked by his sudden death Saturday ... as we first reported, he was found unresponsive in his jacuzzi after playing 2 hours of pickleball at Riviera near his home in Pacific Palisades.

Of course, Matthew's been open about his history with substance abuse, and while there's speculation that could be a factor here, so far there's been no indication that's the case.

For now, Matthew's cause of death remains a mystery ... and our sources have told us foul play is NOT suspected, and there were NO illicit drugs found at his home.

Matthew had picked up pickleball too ... using the sport as a way to keep him away from drugs and on the straight and narrow.