The three main reasons for Dutch cyclists not to wear a helmet are 1) that helmets are considered uncomfortable, 2) that helmets are considered unnecessary, and 3) that they are inconvenient to take along. This is evident from a survey among approx. one thousand adult cyclists ([8]; Table 1). A large share gave other reasons (partly supplementary); mostly that they do not have a helmet, but often elaborating on their view that they do not think a helmet is necessary. The arguments here were that they usually cycle at low speeds, mostly in towns or cities, that they have good bicycle control, and that they mostly travel a known route or a small distance. Elaborations of practical objections included that wearing a helmet results in headaches, that a helmet is too warm to wear, and that a helmet is dangerous because you can't hear anything anymore.

Table 1. Reasons given by Dutch adult cyclists (N=944) for (sometimes) not wearing a helmet (multiple responses possible). Source: [8]
The above information is about adult cyclists. The evaluation of a bicycle helmet campaign aimed at children in the first four years of primary school, running in the province of Zeeland in 2010-2015, provides some information about children [11] [12]. It showed that for both children and their parents, safety is the most important reason for wearing (or being obliged to wear) a bicycle helmet. The most frequently mentioned reason for children to stop wearing a bicycle helmet is that their peers also cycle without one and that parents do not want their child to stand out. Other reasons given by parents for not having their child wear a helmet are that the child is careful and cycles quite safely, and that according to them the route to school is safe enough.
International research shows similar reasons for not wearing bicycle helmets when they are not mandatory: not necessary, uncomfortable, ugly, inconvenient. The research was mainly conducted among children and adolescents, see for example [13] [14] [15] [16] [17] [18].