The reason to believe this lies in the fact that investors' sentiment is still depressed. A good measure of this is the ratio of the Aim index to the FTSE 100. This is because Aim stocks are generally more speculative and less well-known than FTSE 100 ones, and so investors buy them when they are confident but dump them when they are depressed. A low ratio of Aim to FTSE 100 is thus a measure of depressed sentiment. And recently, this ratio has been close to its lowest level than at any time other than in the darkest days of the 2008-09 crisis.
This matters for everyone because sentiment helps to predict companies' capital spending. High sentiment in the tech bubble of 1999 led to an investment boom; the subsequent drop in sentiment led to investment falling relative to GDP; the levelling off of sentiment in the mid-2000s stabilised the investment-GDP ratio, and its slump in 2008 led to falling investment. You'd expect a lag between sentiment and corporate investment simply because it takes time for investment decisions to lead to actual spending, so capital spending this year reflects sentiment last year.