I think the answer has to do with the materials that plants and animals are made of.

Smells are made up of complex organic molecules. Animals have lots of proteins and fat, which, when broken down, can easily form these molecules.

Plants are more... basic in nature. They consist of water, fibres, sugar... and proteins, but never in the amount found in animals.

Bear in mind that the decay processes that produce smell usually break down molecules, not combine them.  In animals they simply have more to work with, to create more 'interesting' left overs.

It mostly has to do with sex and not all plants smell nice.

Look up the number of plants whose botanic name ends in foetida or similar. It means stinky. Recently it was shown that yeast cells on pollen produce the dead meat smell of many stinky plants.

As for the "nice' fragrance of flowers the chemicals are used to attarct polinators. Some plants attract butterflies by smelling like them. (Squash a butterfly one day and smell it!). Very small amounts of fragrance is needed, far less energy is used by the plantproducing fragrance than producing masses of pollen and tossing it about on the wind.

Plant leaves smell nice for different resons, probably chiefly to poison predators or kill fungi and bacteria.

Animal urine is facinating for many animals. Take your dog for a walk and see. The smell of other dogs urine tells if they are male or female, in heat, sick or well. It also helps mark out an animal's territory. So, as it is for beauty, a wonderful fragrance is in the nose of the sniffer. I did have a girlfriend once who loved the smell of my armpits.

BTW the strongest fragrance chemical used by a plant is isbutyl-2-methoxy-3 (6)-pyrazine. One drop will fragrance a swimming pool or two. It is found mainly in chillies.

Michael Bailes

The Fragrant Garden

Australia

While the previous answers are right in terms of chemical components, I think the real answer is reversed. We evolved to assign 'good' and 'bad' to odors based on the effect of the substance on us. Those who routinely found excrement attractive probably died off from various infections faster than those who found it disgusting. Conversely, those who liked the smell of fruit would get more nutrition than those who loath them. 

Note that from the plants point of view it doesn't matter how it smells, as long as it get the job done. A sweet-smelling rose pollinated by bees and a rotting-meat odor that attracts beetles are equally good, even if we don't find them equally attractive

This is a bit like the question 'does a falling tree make a sound if there is no-one around to hear it?' 

To which the obvious answer is 'no, it makes a shockwave.  It only makes a sound when there is someone around to hear it'

Similarly, there is nothng inherently 'nice' or 'repulsive' about a molecule sensed by any organic or inorganic sensing apparatus.  The reaction in a human nose, for example, is determined by the interpretation which, in turn, is determined by the survival value associated with the stimulus received by the observer from the environment.

Thus, there is definite survival value in ascribing a 'nice - safe' label to plant fragrances where there potentially good foodstuffs are to be found - e.g. blossom from the orange tree smells fabulous. 

Conversely, animal excrement or decaying body tissues contain bacteria that will be extremely harmful to another animal whose body is made up of the same materials on which the aforementioned bacteria are feeding.  There is therefore definite survival value in ascribing a 'repulsive - avoid' label to the vapours given off by excrement or decaying tissue.

However, if you are not an avid reader of New Scientist and you are actually a dung beetle then I may have given a slightly rearranged answer......

Not all plants necessarily smell "good" as such. But the reason for why many do is for pollination, and reproduction, it's predominantly (well, all from my knowledge) in insect pollinated plants. They smell nice, have brightly, coloured flowers with large petals and nectar to attract the insects which then pollinate them, by spreading pollen stuck to the insect onto the stigma of the plant which is generally sticky in order to catch pollen. The insects will often drink / eat some of the nectar, brushing past the anthers to get to it. The pollen has adapted and is sticky to an extent and will stick to the insect until it reached the next plant, where some of it will be deposited on its stigma.

You can find some very good resources for this information simply online, and more information on insect-pollinated plants.

Also, many plants contain more organic molecules than animals. For example, esters. Esters are the result of a reaction between a carboxylic acid and an alcohol (not beer but distilled alcohol such as ethanol), even when produced in labs these smell great, they are also used in food flavourings. These are what occur in some instances.

However, in some instances it would perhaps be personal preference, for example some people like the smell of petrol where others don't (like me). I personally like the smell after everything is damp after rain has just fallen and freshly mowed grassed, whereas others may not like those smells so much.