Fructose is mostly converted by the liver and much of the energy obtained from it is used to synthesize fat.
In moderate quantities, fructose should cause no bad effects, but in excessive quantities it causes various problems, like fatty liver, as it has been known for millennia, because the method of producing foie gras is based on this (i.e. overfeeding geese with sweet fruits, but the same method would also work on humans).
For humans it is recommended to avoid a daily intake of more than 25 g fructose or 50 g sugar (sucrose).
When the main source of sugar are fruits it is not easy to eat more sugar than recommended, because most fruits have only around 10% sugar. Only a few fruits have over 15% sugar, e.g. grapes, (fresh) figs and (fresh) dates, and even with those you have to eat generous amounts before eating too much fructose.
On the other hand, with dried fruits or with food with added sugar or HFCS, it has become extremely easy to eat too much fructose.
The effects of fructose can be compared with those of alcohol, even if they are much less severe. In small quantities, both are sources of energy, but in too large quantities they both damage the liver. Fructose does not have effects on the nervous system and it becomes harmful only at higher daily intakes, but nonetheless it must not be abused.