I’m not quite sure which of two questions you are asking here, but as this is basic to understanding metabolism, I think an answer to both questions will be of most general utility, even if you personally already understand the answer to the first.
 The Gibbs Free Energy Change in a chemical reaction determines whether it will proceed spontaneously
If a reaction involves a negative change in Gibbs Free Energy (ΔG), it
will proceed spontaneously; if not, an input of energy is needed to drive it.
The breakdown of glucose to carbon dioxide and water has a large –ve ΔG and therefore does not require input of energy — the reverse reaction has an equivalent +ve ΔG and therefore requires an input of energy to drive it.
This basic aspect of chemical reactions emerges from the laws of thermodynamics, and is explained in detail in section 1.3.3 of Berg et al., available on-line.
 Reactions that break bonds tend to involve negative changes in free energy
Why does the oxidation of glucose have a –ve ΔG, and its synthesis a positive –ve ΔG? This is because glucose oxidation involves breaking covalent bonds, and its synthesis making covalent bonds.
To understand the thermodynamic contribution of bond energies, the account in section 2.4 of Lodish et al. (available on-line) is recommended. In brief, ΔG = ΔH – TΔS, where H is enthalpy and S is entropy. The bond energies contribute to ΔH, and there is also generally in increase in entropy when two moieties become one, as mentioned in the answer by @MangoPrincess.
Not all chemical or biochemical reactions involve net formation or breakage of bonds — for example molecular rearrangements. In these cases more sophisticated chemical considerations are needed to understand the experimentally determined changes in free energy.
The importance in discussing biochemical reactions in terms of Gibbs Free Energy changes is that an energetically favourable reaction can be coupled to an energetically unfavourable one to drive the latter, and it is the quantitation of the ΔG that allow one to predict or understand this. It is also important as ΔG can be quantitatively linked to oxidation–reduction potentials.