Explain why gases do not have their own form and constant volume?

1.Because gases have a very weak connection between its constituent particles (molecules, atoms or ions), they also move chaotically and during their movement diffusion occurs. Gas will have its own shape if it is “closed” or simply kept in one vessel
2.Because:
1.According to the Gay-Lussac law, the volume of a gas depends on the absolute temperature at constant pressure, and the temperature in any place can change, it changes even with the collision of chaotically moving gas atoms, weak of course, but still
2. The gas is rapidly mixed, its density, mass and hence density change.