They are fun to open because Blizzard like everyone in the business has studies psychology and knows that bells and whistles are appealing to people. And they know that tickling the sense of gambling inside people's heads is a good drug to make people want this stuff more.
To me this has the opposite effect, it's not fun because it is frustrating. It is frustrating to slowly earn these boxes, open them and get more shit you don't want at all. Take Overwatch for example, currently there is that Invasion/Intrusion whatever event going on, in that event there was a very cool Torbjörn skin I wanted, however, it can't be bought immediately but has to be earned either in loot boxes or bought with gold. People told me you could earn loot boxes by playing Arcade, and you can, but it's only 4 boxes a week you earn that way. And levelling has slowed down to almost a halt for me, so at best I could get maybe 10 per week if I only played Overwatch during my free time, which I don't want to cause I got other shit to play as well. In the end, in all my free boxes, I didn't get that skin I wanted, so I had to pay money, more than I should have, to buy loot boxes and try and get it (still didn't) and if not use the coins to FINALLY earn that Torbjörn skin I wanted before it potentially goes away forever.
So no, fuck loot boxes. I don't want to care about them, I'd rather pay a medium fee to just get what I want and be done with it cause I am psychologically programmed to wanna dress up my characters in a style I enjoy when playing and by that I mean WHEN I am playing, not towards the end of my enjoyment of the game when whatever I wanted finally drops through some random number generator but I am just about done playing the game.