Our evolutionary past and our dietary habit make the flower pleasing to us.

Our ancestors have been eating flowers and nectar as food and thefore had positive response to the sense of flower's smell. This has been passed down the species as instinct. On top of it all, humans have been eating a large numbers of flowers themselves, thus adding to the overall affect.

Plants have been producing these chemicals or looks to attract the species that help them propogate.

Having said that, I must add, that not all flowers are pleasing in smell or look to humans. The plants produce these repulsive chemicals or looks to actually deter animans or othe species from destroying their flowers.