Tumblr is a platform where people get to share their thoughts via pictures. Most of the people on the platform are actually the outcasts who aim to transmit their thoughts by writing posts. Naturally, it seems as if the place is full of lonely and depressive posts.
One kind of group attracts similar people and so other people go on the site to look for like minded individuals. Moreover, coming up with touchy words and quotes is considered to be something of an art. People create these posts on purpose to kindle the art, as they call it, which may make the platform seem depressive at first sight.
On the surface, it looks like tumblr is full of people who are struggling with depression and anxiety. It includes people who consider themselves as an outcast of the society. Not only that, but it also hosts a good number of users who consider themselves as aesthetes and will go to any extent for art.
Consider it like a trend mixed with the snow-ball effect, due to which youngsters struggling with stress/depression/anxiety easily get attracted to it due to the overall theme of the website and for the community of like-minded people that they could find easily there. This is what attracts them.