Why do humans prefer symmetrical arrangement of objects? Most of the times, we associate symmetry with beauty. The symmetry may be in architectural/interior design for instance. Why would this be so ?
Indicator of genetic fitness argument

There is an evolutionary psychology argument. As with most evolutionary psychology arguments, the strength of the evidence is typically a bit fuzzy.

Symmetry in many aspects of the human body is functional. Such symmetry might be seen as the natural state that arises from a healthy life and a youthful body. In contrast various genetic abnormalities, diseases, and the like  can give rise to asymmetry (e.g., scars, moles, freckles, ageing processes, deformities, etc.). 

The argument might continue that it is adaptive for us to seek out sexual partners  who appear genetically and environmentally fit where symmetry may be one indication of this fitness. One could even extend the evolutionary argument to suggest that it would be adaptive to avoid certain types of diseased individuals in order to reduce the risk of catching some disease, where various forms of asymmetry may be indicative of this.

How does this explain our desire for symmetry in physical objects? The perception of beauty in the environment might be seen as an extension of perceptions of beauty in other people. 

Little and Jones also summarise this perspective


  One explanation for the preference for symmetrical faces comes from a
  postulated link to an evolutionary adaptation to identify high-quality
  mates (see Thornhill &amp; Gangestad (1999) for review). Symmetry in human
  faces has been linked to potential heritable fitness (‘goodgenes’)
  because symmetry is a useful measure of the ability of an organism to
  cope with developmental stress (both genetic and environmental). As
  the optimal developmental outcome of most characters is symmetry,
  deviation from perfect symmetry can be considered a reflection of
  challenges to development. Only high-quality individuals can maintain
  symmetrical development under environmental and genetic stress and
  therefore symmetry can serve as an indicator of phenotypic quality as
  well as genotypic quality (e.g. the ability to resist disease: see
  Møller (1997) and Møller &amp; Thornhill (1998) for reviews). This logic
  would lead to a preference for high symmetry mates as evolution will
  have favoured individuals who had preferences for high-quality mates
  over low-quality mates. Indeed, morphological symmetry appears to be
  related to reproductive success in many species, including humans
  (Gangestad &amp; Thornhill 1997a; Møller &amp; Thornhill 1998). For example,
  more symmetrical human males have more sexual partners than less
  symmetrical men (Thornhill &amp; Gangestad 1994) and symmetrical males are
  also more likely to be chosen as extra-pair partners (Gangestad &amp;
  Thornhill 1997b). Thus the link between symmetry and attractiveness
  may reflect that preferences for symmetrical individuals may be
  potentially adaptive.


Perceptual argument

Enquist and Arak (1994) articulate a perceptual clarity argument. They wrote (my bolding):


  Humans and certain other species find symmetrical patterns more
  attractive than asymmetrical ones. These preferences may appear in
  response to biological signals1–3, or in situations where there is no
  obvious signalling context, such as exploratory behaviour4,5 and human
  aesthetic response to pattern6–8. It has been proposed9,10 that
  preferences for symmetry have evolved in animals because the degree of
  symmetry in signals indicates the signaller's quality. By contrast, we
  show here that symmetry preferences may arise as a by-product of the
  need to recognize objects irrespective of their position and
  orientation in the visual field. The existence of sensory biases for
  symmetry may have been exploited independently by natural selection
  acting on biological signals and by human artistic innovation. This
  may account for the observed convergence on symmetrical forms in
  nature and decorative art.


References


Enquist, M., Arak, A. &amp; others (1994). Symmetry, beauty and evolution. Nature, 372, 169-172.
Little, A.C. &amp; Jones, B.C. (2003). Evidence against perceptual bias views for symmetry preferences in human faces. Proceedings of the Royal Society of London. Series B: Biological Sciences, 270, 1759-1763. PDF