A lot of "friendliness," (more accurately, "handleability") depends on regular handling. While some species are more naturally flighty than others, they all will -- to varying degrees -- become more used to hands/handling if they're exposed often enough and gently enough. I actually wrote out a little hand-taming-tips for lizards a short while ago, so I'll paste it here. In general, frogs aren't as "chill" with hands as lizards can be, but over time, this could probably work. Most of the anurans I've kept were fairly handleable over a long enough time-line.

>>Step 1: Place hand in tank, without moving it, for ~10 minutes a day or as long as you can comfortably do this. Repeat until animal seems "ok" with this. (No visible signs of stress.)
Step 2: Move hand to animal. Try to gently scoop if from below. If the animal is very shy, spend a few days just putting your hand under its tummy, without lifting, to get him used to that until he's ok with that. (Try 2 hands if it is already too large for 1.)
Step 3: Pick animal up -- preferably by "scooping" if from below. Hold it for 10-15 mins a day until he's ok with that. [*As I look this over, I think I would shorten contact for amphibians, because their skin can dry out, and that's dangerous -- maybe cut all these #s in half and keep close eye on their moisture needs?]
--In theory, after the completion of step three, your animal will be ok with being handled respectfully when it's playtime. The more often you have contact, the more comfortable s/he'll be with handling.

NEVER "chase" an animal around the cage (except, perhaps, for a medical emergency.) This teaches them to equate hands with fear. (You take on the role of "predator" on an instinctive level.) It's harder to re-train an animal once it's afraid of hands. (I'm going through this now with one of my parakeets, whom I had to grab and terrify to get a claw unstuck from a snuggle sack. She HATES my hands right now, even though they did not scare her before that moment.)

Keep in mind that many animals will, generally, fear a hand coming from above more than one from below, as birds are often their predators, so it is instinctual. It's hard not to "come from above" when reaching into a tank, but this is why "scooping" (hand itself is fully below animal) rather than "grabbing" (hand from above grabs from above) can help. Once an animal is used to you, it might learn not to mind being grabbed from above. Again, individual temperment comes into play. It is very possible for to members of the same species with shared past experiences to feel very different about hands for no discernable reason.

In a general way, these rules can apply to most pets that can be hand-tamed. (I actually learned the "no chasing" rule while researching hamsters, and then saw it again regarding birds. It seems, though, to apply to many species that are prey food in the wild.)


*I should also add that some of the bigger species that would naturally eat food items as large as your fingers might be poor choices for trying to make handleable, since they'll probably think of you as food no matter what you do. I wouldn't really try to "hand tame" a pixie or any of the species sold as "pacman" frogs. It would be like training a dog to walk on a leash that's made of bacon. Just ain't gonna work out too well. I guess maybe if they're well fed and trained from a young age? I'm actually becoming more and more curious to see what people consider a "handleable" frog to be, and what experiences they've had in the past. I know that many of us consider frogs eyes-only pets, because their skin is so sensitive, and there's so much that can go wrong with an amphibian on an excursion outside of the enclosure. (I have a nagging fear that, someday, one of my frogs will jump into a cup of coffee that I have nearby.)