I was raised in catholic schools, and I always played a little game with myself while trying to not fall asleep or fart or talk during mass...

I'd see how well I could follow along with the singing without the hymnal (book). Sometimes teachers or my mom would force me to open it up and follow along, but I never felt like I needed to...

Without putting too much thought into it, I'd guess it has something to do with knowing that even if I mess up, there are a hundred other people singing, so no one would notice my mistakes. But I did always find it a little weird or neat that I could do it in the first place, and never asked anyone if they felt the same way about their own singing...

who knows

4 options:

1) You've heard the hymn at some point in your life and just don't remember but due to the nature of music, it comes back easily.

2) Many hymns do follow patterns. Maybe you're just really good at pattern recognition.

3) You are tapping into the collective unconscious, in which the emotional use and long history of hymns (vs. pop songs) makes it much more deeply ingrained and thus easier to access. (common to everyone)

4) They are memories derive from a reincarnated past life, in which hymns played an important part, or from a genetic memory passed down by an ancestor (person specific)

Most of the old hymns were somber as a reflection of the puritanical influence.... respectful, humble, sense of humility in the presence of the Lord...with no worldly decadence...purely practical.

Further, most of the old southern hymns were meant for an uneducated populace...so all have the same notes and rythmic patterns...just arranged differently.

Although there are somber hymns...there are some real picker uppers...Bringing in the Sheaves, When the Roll is Called Up Yonder I'll Be there, Let Us have a Little Talk With Jesus, What A Day That Will Be, Leanin' On The Everlasting Arms...

Oddly enough, many of the old hymns can be sung in time to "chopping a garden" or "hoeing corn." Perfect rythm to rocking in a rockin' chair....while rockin' a baby to sleep...or shelling beans.

Many of these hymns are so ingrained into some cultures, that you probably have heard them and didn't even know it...especially if you live down here in Dixie.

Having the misfortune to have gone to a school where the hymn were a daily occurence, i can say your ok and this is not some massive conspiracy.....

Most hymns seem to go along with one of four or five melodies

Also seeing as that has made you wonder i better clear this up for you as well...
when you walk up to automatic doors they open by being activated by a pad or sensor,,, youre not really a jedi

Brother, in order to respond your question I quoted and edited you. In your own words :

Originally posted by woogleuk
I'm watching a Christmas service on TV and I started singing along,

Anyone else have a similar experience? Anyone have an explanation as to why?

Nope.