Why are high/rising property values considered a good thing?
Considered a good thing by whom?


Financial institutions and speculators: yes, since houses for them are just another asset. Higher asset prices is a key mechanism of profit making for financial institutions (capital gains). In the recent financial crisis house prices collapsed and people defaulted on their mortgage payments. Thus,  the fall in the value of houses repossessed by banks meant the return on financial assets associated to housing translated into massive losses for financial institutions and government agencies associated with the housing market like Fannie Mae and Freddie Mac in the US.
Home owners: partly yes. A price increase is good, particularly if you have a mortgage to pay (so if you sell the house you can pay the loan back). Rising house prices are also good for pensioners, which main source of wealth is their house. Rising prices also give people the feeling they are richer (and hence spend more). Some argue that the positive appreciation of the media of rising house prices is partly due to the fact that newspapers are written and read mainly by older people, not by youngsters i.e. renters.
Renters: no, because rents are associated with house prices. If incomes are not growing at the pace of house prices, renters will be squeezed, to the benefit of landlords and homeowners. This is true in the UK (and in many other advanced economies, see here):





Central banks, and perhaps the overall economy and population: not necessarily, because rising house prices fueled by speculation and indebtedness (e.g. speculators and investors entering into the buy-to-let market) mean possibilities of a bubble bursting and a subsequent financial crisis. For example, in the UK, Mark Carney has expressed such concerns in the past.
Estate Agents: yes, as their business as intermediaries depends partly on fees from sales. People is more willing to sell when the market is on the up, fostering the property chain. If prices are falling, people prefer to wait instead of selling their house, breaking the property chain.