One of the things that people pick up on subconsciously is whether or not your body seems to be in tune with your reactions, whether it all moves "together", in the same way a smile is probably fake if it's not involving the eyes and the rest of the face. So if your general body language and movement seems to be positive and sings along with your words and facial expressions, it comes across as genuinely "happy". It's hard to fake this sort of ensemble - unless you're an actor - so maybe you are just exuding a happier healthier sort of vibe all round.
Not a body language expert myself, but I started looking into the subject and found that the opposite of this is probably what me made me appear very unhappy or even intimidating to people in the past - a stiff way of holding myself, and verbal reactions that seemed disembodied from my general body language. This unnerves people. They probably aren't thinking analytically when they see it, but their subconscious is picking up on it. Came from years of feeling awkward in my skin and holding myself carefully together around others... even my therapist commented on it last time I saw him, how stiffly composed and distant I seemed to him, and my body really was not "singing" with my words, it was what they call 'guarded' or 'gated' body language. It's quite normal for me, but for the average person, it seems odd to them and suggests someone ill at ease or extremely aloof, possibly even someone hiding something or being dishonest (because when analyzing people's body language, stiffness of the body while speaking tends to indicate lying, as does lack of eye contact - except with me, I maintain eye contact almost all the time, which unnerves people even more).
Anyway, I imagine you're just feeling more comfortable on multiple levels and your body will broadcast the fact.
