I often joke around with my girlfriend that ever since we started dating that I no longer have custody of my arms, as whenever we're together she's hanging on to one or the other in various situations. (This is a play on the idea that in a marriage mine is yours and yours is mine with the joke being we're not married yet but she's already got my arms). when she's cold, when she's tired, when we're walking or sitting, etc. I should clarify I don't mean to make out that she's clingy or anything. Not every moment is spent arm grabbing- just that when we are physically intimate, that's her go-to place seemingly regardless of the situation. I'm 6'3 and 76 kg, but my legs are much more developed than my arms due to a history of playing soccer, so I wouldn't think it's down to my impressive musculature, or lack thereof. It's also a very common sight to see couples walking together with the girl sometimes hooked in at the elbows or both hands one above and below the elbow (as in the picture) so I know it's not an isolated case. It's actually kind of a weird position as you end up with one arm kinda just hanging there by your side and the other one's still swinging freely (when walking).
I'm guessing it's something psychological/practical like a sense of security or something, but I'm curious to see what other reasons could be.