In other words: whereas effort comes from the person, joy flows from Heaven.
Of course, G-d can imbue a person with joy without any effort on the person's part. However, such joy would remain impersonal and disassociated from the individual - it would be superficial and thus not real joy. On the other hand, if one strives to progress and feels no joy in response, he loses the motivation to persist. In the ideal state, joy is granted to one who strives to grow and triumphs. For such joy is both associated with one's own accomplishments and also offers incentive for continued advancement. This is signified by the interpenetration of the two triangles - the 'Star of David'.
On the underside of a skateboard are two trucks [metal frames with two wheels on each]. Looking carefully at their shape we discern two triangles. The truck near the tail/bottom end of the skateboard points upward, while the one near the nose/top, points downward. For me, this serves as a reminder of the above mentioned benefit of skateboarding: it can serve to shield off - and ultimately conquer - languishment.