In What Key is The CC-A-D Tuning?
General mountain dulcimer or music discussions
Dusty Turtle is right. (And if you have never tried playing in DAG, it is interesting!)
I think we hear both Aeolian and Dorian modes as "minor" because in both, the 3rd note of the scale (the third scale degree, in theory talk) is a minor third (3 semitones) above the key note (root). (This is the F natural at the 3rd fret in DAc.)
Aeolian is the "natural" minor scale. Dorian is a little "brighter" because it has a raised 6th note. If your instrument has a 6+ (6 1/2) fret, you can hear the difference. Here's how:
If you play in DAc using the 6 fret, the melody string(s) give D Aeolian. Starting with the open string (C) and going up to the 8th fret (skipping 6+) you get: C D E F G A Bb C D
DAc using the 6+ fret gives D Dorian: C D E F G A B C D
In arrangements of Dorian melodies in D, you often hear the B natural harmonized with a G major chord (ex.: 7-6-6+ or 3-3-6+), which can be a nice contrast to the minor harmony implied by the F natural.
Hope that illuminates - TomR